The New World crops that spread globally during the Columbian Exchange had been domesticated for thousands of years, with thousands of years of selective breeding to make them better suited for agriculture. The Old World crops (and animals) that went the other way, to the Americas, also had thousands of years of improvement through selective breeding behind them. This makes a big difference - if, instead of domesticated maize, only teosinte was available, it would not have been as widely adopted in Europe, Africa, and Asia. The New World crops that spread quickly were to a large extent alternatives to existing crops, providing better yields under suitable conditions. For example, the peanut spread through Africa as a new version of the bambara nut and China as a new oilseed, cassava spread through Africa as an alternative to yams, and SE Asia as an alternative to taro. The potato and sweet potato were alternatives to turnips, taro, and yams. New World beans (mostly Phaseolus vulgaris) spread as alternatives to broad beans, lupins, and other Old World beans. The chilli was a culinary alternative to black pepper, and could be grown where black pepper could not (and needed to be imported). These crops were not adopted in Africa, Asia, and Europe just because they were different - they were adopted because they produced higher yields. The conditions under which New World crops produced higher yields in the Old World affected where they were adopted. Where the older crops performed better, the older crops remained.

"Higher yield" needs some qualification for subsistence farming. Subsistence farmers generally don't seek to maximise their average yield, but rather to maximise their minimum yield. It does little good to eat well for 5 years and then starve to death in the 6th when crops fail. Subsistence farmers often make use of techniques that reduce their average yield but improve their food security (i.e., maximise their minimum yield), such as growing crops on dispersed plots (which is less efficient, but reduces the risk of pests and disease, flooding, etc.) and growing multiple varieties (which reduces the risk of pests and disease and the impact of poor weather). Thus, in Africa, after the adoption of New World taro, cassava, and peanuts, New World and Old World taros were both still grown (often next to each other), bambara nuts were still grown alongside peanuts (despite their lower yield), and taro and yams were still grown even where cassava yields were higher. However, while this means that absolute higher yields aren't required for a new crop to be adopted, the yield must still be sufficient compared to existing crops, and undomesticated plants have little chance of being successful in this competition.

It is this lack of high-yield domesticated crops that reduced the global impact of Australian plants on farming, and led to farming in Australia being thoroughly dominated by imported crops. Australian undomesticated cereals, legumes, and root crops could not compete yield-wise with the combination of New World and Old World crops already being grown. The lack of domestication meant that fruits were usually small and sour. Small made them less efficient to pick, and sour made them less consumer-friendly. Any possibility of success of Australian crops in Europe was hampered by them being adapted to Australian conditions. Just as some plants brought to Australia by European settlers did poorly, being unadapted to Australian condition, Australian plants could and did suffer from a similar problem if taken to Europe.

Some Australian plants are cultivated as foods. The majority have a very short history as crops, with the modern "bush food" industry only beginning in the 1970s. Significant plants include the lilly pilly (riberry) and lemon myrtle. Much of the commercial lilly pilly production is made into jam, and lemon myrtle is a spice. Both are marketed as gourmet bush foods, and would have great difficulty competed as generic foods without that "bush food" status.

The exceptional Australian crop is the macadamia, commercially grown in Australia since the late 19th century, and introduced into Hawaii in the 19th century, and commercially grown there since the early 20th century. The macadamia avoids most of the non-domesticated problems. Nuts are typically much less modified by domestication than fruits - the hard shell allows the wild varieties to be large enough, and nuts are not grown for their sweetness. A disadvantage of macadamias compared to other nuts is that their shells are very tough and hard to crack. Modern processing machinery allows shelling to be automated, and the nuts can be sold shelled. The macadamia does suffer commercially from being adapted to Australia, which has meant, for example, that is hasn't been grown commercially in Europe. Outside Australia, it grown in the largest amounts in South Africa, Brazil, Hawaii, and Central America. The global food trade has enabled the macadamia to succeed commercially as well as it has - if consumption was only local, it would be a much less important crop. The macadamia had good timing: machinery for processing the nuts was available, and the global food trade could carry it to markets around the world.

In summary, there were no domesticated plants available in Australia when Europeans settled there. Some plants have been domesticated since then, but mostly only as minor gourmet crops. The exception is the macadamia, which has become an important crop, but far from a staple like maize, potato, cassava. As far as tree nuts go, the macadamia is the 7th biggest tree nut crop, but with an annual production of about 60,000 tons (kernels) compared to over 4 million tons total for tree nuts (and tree nut production is in turn dwarfed by peanut production, averaging about 40 million tons per year (in shell)).