I assure you that the government has in-house lawyers who check for unconstitutional provisions at the earliest and the drafting stage of any proposed legislation. Possible breach of s.92 of the Constitution would be among the matters they would look at.
What you have to consider is that the cash limit doesn't restrict the transaction itself, only that it cannot be conducted in cash.
The transaction can proceed by various means including electronic transfer. In this way there will be a full record of the transaction.
The idea seems to be to clamp down in the black economy. So to argue this you'd have to argue that there would be an effect on legitimate interstate trade.
There might be an effect on illegal or illegitimate trade, such as the drug trade. But is that going to be within the matters intended to be protected by s.92? Or trade that is seeking to avoid taxation.
I'd suggest it is implied in the Constitution that only legitimate trade is intended by the term. So, I'd suggest the answer to the issue, is this is likely to go ahead.
