Where does the Aussie expression, "What do you think this is, bushweek?" originate?

The bushies, of course, are supposed to be a bit slow on the uptake (and perhaps as thick as a brick) so lots greasy larrikins and city slickers try to rip them off. Thus the expression bush week is used ironically by someone who suspects they’re being made the victim of a scam or prank, as in the question “What do you think this is, bush week?”
abc.net.au/newsradio/txt/s1512909.htm webpage no longer exists

The pioneers who had cleared the land were often perceived as dim witted yokels by their city slicker cousins. This was made very clear in 1918, when attempts were made to organize an event in Sydney to be called Bush Week ... It should have been easily possible to have secured a better name for the “week” which is going to be conducted in Sydney for the purpose of advertising that part of New South Wales which is outside the city. The name “Bush” week is a survival of the old days and has little or no application now. On the other hand the term Country Week would be much more suitable and fitting ... Unfortunately the event was boycotted by people from the bush. Having learned their lesson, the promoters renamed it Country Week the following year, but it was never a great success. The term, ‘What do you think this is, Bush Week?’ slowly crept into the Australian lexicon. In some areas it was a reference to naive country bumpkins visiting the city and being taken advantage of. It was used in the context of, ‘What do you take me for, an idiot?’

https://paulineconolly.com/2016/what-do-you-think-this-is-bush-week

True, and to which many people reply because they think its funny, "No its Forrest Fortnight"!