Short answer:

The question has mistaken assumptions. Aboriginal Australians did have outbreaks of disease which closely parallel what happened in the Americas. The Japanese, while politically isolated, were not epidemiologically isolated-- Japanese had travelled to China and Korea for a very long time, and so the "arrival" of Europeans in the modern era did not bring with them the same kind of devastation. Japan had had prior "virgin soil" smallpox epidemics, which had been devastating-- but that was a thousand years in the past. The most challenging new arriving disease was cholera, which _did_ produce substantial loss of life, but which was ultimately controlled by effective public sanitation and hygiene.

Discussion:

Disease in Aboriginal Australia

In SMALLPOX IN CENTRAL AUSTRALIA: EVIDENCE FOR EPIDEMICS AND POSTULATIONS ABOUT THE IMPACT, R. G. Kimber summarizes the literature

a) There were at least two epidemics, that of 1789 and 1830, which had a devastating impact on the arid land populations;

b) many people died because no traditional remedies had any effect;

c) the epidemics came from the north, one of the epidemics beginning at the mouth of the Adelaide River; and

d) even Aborigines in areas that might be expected to be remote from wider contact, such as the Rawlinson Ranges, suffered the full impact of the epidemics.

Smallpox appears to have been just about as deadly in indigenous Australia as it was in the Americas:

The case fatality rates possibly reached 75 per cent in virgin soil epidemics, whether in 1789 or the early 1830s ... A 45 per cent fatality rate might apply to populations suffering a second outbreak of smallpox in the 1830s; however, the many deaths of children and women, sometimes for the second time in 40 years, created demographically exceptional circumstances

The indigenous Australian experience with disease thus looks pretty similar to the indigenous American experience. There _are_ significant outstanding issues in the history and demographic impact of epidemic disease on indigenous Australian populations-- was smallpox introduced by Europeans, or "Makassars" -- from Sulawesi in present day Indonesia is a notable one.

But that indigenous Australia experience devastating epidemics in 18th and 19th centuries is well established.

Disease in Japan

Japan had had smallpox for a very long time-- the first recorded smallpox epidemic is in the 8th century CE:

The first recorded smallpox epidemic in Japan was in the eighth century. The smallpox that started in 735 ravaged the country and killed probably about one-third of the entire population. Almost certainly this was a virgin soil epidemic. Later, twenty-eight smallpox epidemics were recorded until 1206. Among these epidemics, there was a clear trend of progressive shortening of the interval between two epidemics: until the year 1000, smallpox visited Japan with the interval of twenty-four years on average, while between 1001 and 1206 the interval became thirteen years.By the Tokugawa Period or the early modern period in Japan, smallpox was firmly settled as an endemic disease. Statistics from a village show that the village experienced major outbreaks of smallpox about every ten years. They also show that about ninety-five per cent of the deaths from smallpox were those who were under ten-years-of-age.

So by the modern era, Japan had an experience with smallpox a lot like Europe's -- it was a disease of children, and adults were likely immune. Contact with Europe brought techniques of immunization for smallpox, which the Japanese adopted very quickly, and to great effect; rater than introducing the disease to Japan, modern European arrivals brought the prophylaxis and the Japanese began to vaccinate their population against smallpox very quickly

The Japanese government in the Meiji era decided that all Japanese people should be immunized with the vaccine for smallpox, which was stipulated in 1876, and a vaccination law against smallpox started in 1910.

In the 19th century, Japan was perceived as a nation with a heavy burden of disease-- cholera, plague, smallpox, typhoid, yellow fever. The Japanese rapidly developed an effective public health system; the arrival of Western medicine brought medical technologies which substantially alleviated and existing burden.

The most important "new" disease that appears to arrive with contact with the West is cholera, which produces significant epidemics in 19th century Japan. Cholera can be controlled with good public health measures, and the highly organized Japanese state readily learned and applied these lessons, more or less at the same time that Europe was confronting the same issues -- the 1854 London Cholera epidemic is roughly the same time, and its lessons reach Japan quickly.

​

Sources

Australia

Kimber, R. G. “Smallpox in Central Australia: Evidence for Epidemics and Postulations about the Impact.” Australian Archaeology, no. 27, 1988, pp. 63–68.

Macassans and Aboriginal Smallpox: the 1789 and 1829 Epidemics

SOME MYTHS IN THE DEMOGRAPHY OF ABORIGINAL AUSTRALIANS

Invisible Invaders: Smallpox and Other Diseases in Aboriginal Australia, 1780-1880. Melbourne University Press, 2002

Japan

Smallpox and the Epidemiological Heritage of Modern Japan: Towards a Total History

Smallpox and Vaccination in Japan

Vaccine chronicle in Japan

Epidemics and Mortality in Early Modern Japan (Princeton, NJ: Princeton University Press, 1987)

The Vaccinators: Smallpox, Medical Knowledge, and the ‘Opening’ of Japan (Palo Alto, CA: Stanford University Press, 2007)

Cholera epidemics in Kanagawa

The Shifting Epistemological Foundations of Cholera Control in Japan (1822-1900)

A rivalry of foulness: official and unofficial investigations of the London cholera epidemic of 1854.

Amp has essentially answered the question, but I thought I'd expand a little.

Disease had an enormous impact in Australia - darists of the First Fleet recorded that smallpox (or possibly chickenpox) killed nearly half of the Sydney area's Aboriginal population in 1789, and a later epidemic in 1836 killed two thirds of the Dungog district in NSW. In the frontier wars that plagued Australia from 1788 to the 1930s, war with Indigenous tribes often came to end when the native population had been so devastated by disease that war could not longer be maintained. Wars also often started because unexplainable deaths were often blamed on curses and magic, aimed at them not just by settlers but also traditional rivals, meaning European disease set off wars between neighbouring tribes.

The economic historian Noel Butlin argued in his book Our Original Aggression that disease may have been deliberately spread by the British, but most evidence suggests otherwise. David Day in Claiming a Continent argued the same, implicating a Major Ross of the Royal Marines, who seemed eager for a fight as he had insisted that fortifications needed to be built and decisive action needed to be taken against the (mostly peaceful) local Eora people. Day claimed that Ross had seen the effects of disease on native populations during the Seven Years War in North America, and thus was a likely culprit, but there is no evidence actually implicating him or anyone else.

Other common diseases included influenza, the cold, pleurisy, cholera, dysentery, measles and several forms of venereal disease. French explorers accidentally brought tuberculosis to Tasmania when visiting in 1802, and measles killed half of the York District of WA in 1865. Veneral diseases were the main killers in King Sound and in Port Phillip Bay.

Major population centers in precolonial Australia later became major cities in colonial Australia, where intense port traffic meant the constant import of diseases. The first 65 years of settlement in the south-west of WA saw serial epidemics hit repeatedly: four flu epidemics, three smallpox and chickenpox, two typhus, whooping cough and measles, and one typhoid. Monop, an old Nyungar man and one of the last survivors of his tribe told Daisy Bates that "the smell of the white man kills us" - but a tribe need not have met Europeans to be afflicted by it.

The widespread introduction of alcohol in its hardest forms led to severe addictions and weakened immune systems. Wet shirts or blankets, which Indigenous Australians were unfamiliar with, helped disease kill. The radical shift in diet to less diverse and nutritious (but convenient and calorie-dense) European foods, used as bribes or rations (sometimes poisoned), and the destruction and theft of traditional food sources, also contributed to weakened immune systems.

The end of nomadic life, and being forced to live alongside livestock, meant that new methods of hygiene, like washing clothes, washing hands and using toilets, had to be learned. Most noticably, disease and malnutrition also brought about a severe decline in birth rates - colonists commented that they found few or no children amongst local tribes. The Bunurong tribe from land near Melbourne went from 300-500 people to 26 in two decades.

Even when European invaders were sympathetic and treated locals with respect, locaks often 'withered away'. Disease predominantly hit the old and wise, or the young who were expected to inherit knowledge and leadership - the survivors found that their society had collapsed.

Exile and imprisonment became a common means of punishing resistance in frontier expansion in Western Australia, and those men shipped to Rottnest Island from across WA were stuffed into crowded prison cells and quickly died. Rottnest became internationally famous as a hell-prison, a far cry from its modern tourist-oriented Instagram-friendly persona.

Death away from country was a particularly upsetting fate, as the person's spirit would remain lost. Aboriginal people who tolerated missionaries and their religious schooling soon withdrew their children when they began to die in large numbers, and they generally feared hospitals as places of evil spirits and foul magic.

Witnessing massive population decline in the Indigenous population, this experience mixed with religious fervour and emerging sciences in colonial society, and encouraged the belief that 'the Aborigines were doomed to extinction'. Replacement by European civilisation was seen as natural, a result of evolution, which was taken as a fact by most Australians and effected the ways in which they interacted with Indigenous survivors. Instead of attempting to ensure their cultural survival, colonials with sympathy instead tried to isolate 'full blood' populations and assimilate 'half-caste' (meaning mixed ancestry) children, leading to a whole new nightmare called 'the Stolen Generations'.

Sources:
Geoffrey Blainey's The Rise and Fall of Ancient Australia

John Connor's Australian Frontier Wars

Neville Green's Broken Spear (which also has several tables listing the epidemics known to have afflicted the Nyungar)