Why is it that Canadian federalism has become very decentralized despite Canada being quite centralized upon Confederation? And why has Australian federalism become more centralized (even more so than American and certainly Canadian federalism) despite Australia starting out as a decentralized one upon Federation?
In other words, why have Canadian and Australian federalisms each gone the opposite of their intended ways?
Canada decentralized so much thanks to the actions of Ontario's premier, Sir Oliver Mowat (premier 1872—1896). He was a very strong believer in provincial sovereignty and argued vehemently that the British North America Act, 1867 was created to ensure the protection of provincial rights.
Mowat was instrumental in winning several groundbreaking appeals before the Judicial Committee of the Privy Council, which was the highest court of appeals in Canada until the 1930s and 40s (it was phased out slowly over time). Several important rulings led to an increase in provincial authority and a decrease in federal control.
Citizen's Insurance Co. v. Parsons significantly affected trade and commerce. This was a big victory in Mowat's championing of provincial rights, which has had a huge influence on Canadian constitutionalism. Future trade and commerce decisions before the JCPC were no longer found in favour of the federal governments for decades after.
McLaren v. Caldwell gave provincial jurisdictions in matters of local nature areas and upheld the province's right to deal with civil and property rights. Mowat had initially attempted to pass the River and Streams Act, 1881 to legislate how rivers and streams could be used. The federal government disallowed the passage of this legislation. Sir John A. Macdonald, a staunch centralist, argued the act infringed upon private property rights. After the ruling by the JCPC, the Ontario legislature re-enacted the bill in 1884 and passed it.
Hodge v. the Queen, probably one of the more famous rulings, established a Canadian convention known as Double Aspect, which allows both the federal and provincial governments to create laws regarding the same subject matter. The challenge involved a man accused of operating a billiards table at the same time alcohol was being served.
Mowat also oversaw disputes in other areas including timber and mineral rights. Despite Macdonald's attempts to create a specifically centralized government, in light of his fears that the US' lack of centralization led to its civil war, Mowat did all he could to hinder that progress. In summary, Mowat and some of the JCPC's rulings are the reason Canada is one of the most decentralized federations in the West.
I know very little about Australian history, so I'm afraid I can't address why it's become more centralized over time. I hope it answered your question as to why Canada has decentralized though.