In the famous Australian folk song "True Blue" written by John Williamson, he asks "If they sell us out like sponge cake, Do you really care?" Is this referring to anything in particular? For example, was the recipe for lamingtons sold off to foreigners?
"Sponge cake" could well be an allusion to small, significant commodities that which can be mass produced and discarded like a Lamingtons. The words "sponge cake", here could have a similar meaning to the British usage of "trifles" when referring to trivial items.
There's no record of the recipe of Lamingtons being sold; this website purports to contain a recipe dating back to 1900: https://trove.nla.gov.au/newspaper/article/101452826
The reference to "sponge cake" is most likely included to provide a syllable sound similar to "smoko", "cockatoo" and Dinkum". These sounds are not intended to rhyme, but to provide a mild stress on the syllable during the song's delivery.
He's referring to our politicians selling out our country to foreign interests like China, which they are.