The thing is, the au pair arrangement was established legally in Australia... families are mobile and take holidays. Requiring work visas for au pairs is small minded in my view as this is hardly displacing new zealand workers.
I think this is more a case of bullying than upholding NZ law.
Thinking more broadly, any such rules would have to apply to any country. If you treat one country differently to another, that raises thorny issues as well.
But even more generally - for example - let's say some tourists start arriving with 19yo 'au pairs' who then spend a week or three working in a brothel while the family is "on holiday". Maybe that's already happening. Maybe that is what is behind this. It could just as easily be a cover story for human trafficking. Customs are on to single young women arriving for this purpose.....and we see on TV how many are sent back (rightly or wrongly).