Why is a tablespoon defined as 20mL in Australia?
The following snippet from Wikipedia doesn't fully answer your question, but it's at least suggestive of what might have happened: the UK standardized the tablespoon as a volume measure using its “eating spoon” meaning while Australia used its “serving spoon” meaning.

Before about 1700, people generally brought their own spoons to the table. Spoons were carried as personal property in much the same way as people today carry wallets, key rings, etc. From about 1700 the place setting became popular, and with it the &quot;table-spoon,&quot; &quot;table-fork,&quot; and &quot;table-knife.&quot; The 18th century witnessed a proliferation of different sorts of spoons, including the tea-spoon, coffee-spoon, dessert-spoon, and soup-spoon. In the UK, the dessert-spoon and soup-spoon began to displace the table-spoon as the primary implement for eating from a bowl, at which point the name &quot;table-spoon&quot; took on a secondary meaning as a much larger serving spoon. At the time the first edition of the Oxford English Dictionary was published, &quot;tablespoon&quot; (which by then was no longer hyphenated) still had two definitions in the UK: the original definition (eating spoon) and the new definition (serving spoon). By the time of the second edition, the first definition was relegated to &quot;also, occasionally&quot;. However the term &quot;tablespoon&quot; referring to a serving spoon has been on the decline in the UK since cooking books became common; it has more and more been used the same way as in the USA due to the measurement system outlined below.
Source: http://en.wikipedia.org/wiki/Tablespoon

Here's a list of the definition of the tablespoon measure from older recipe books published in different English-speaking countries. It's a short list, but it does somewhat show that the Australian definition evolved to be generally higher (20ml-25ml) than the UK one (around 18ml), with the South African one on the lower end (12.5ml).


Margaret Powell (British) 1970 Tablespoon = 18 ml
Australian and New Zealand Complete cooking 1973 = Tablespoon = 25 ml
Best of Cooking (Hamlyn) - Tablespoon, (Australia) = 20 ml
Best of Cooking (Hamlyn) - Tablespoon, (British) = 17,7 ml
Best of Cooking (Hamlyn) - Tablespoon, (America) 14,2
Indian Cooking, Chowhary 1952 - = Tablespoon = 25 ml
Cook and Enjoy, De Villiers 1971 (South Africa) = Tablespoon = 12,5 ml
Complete South African Cookbook (South Africa) 1979 Tablespoon = 12,5
The Australian Women's weekly 1978 = 20 ml

Source: http://whitegranny.blogspot.com/2008/11/beware-of-tablespoon.html

The proliferation of spoons mentioned in the Wikipedia snippet above is still evident in recipe books today. British recipes occasionally still use a “dessertspoon” to mean a 10ml measure. In Dutch recipes (link to a page in Dutch), a 5ml measure is referred to as a “coffeespoon,” while a “teaspoon” refers to a 3ml measure.