Firstly, if they don't have the money, they go bankrupt, so the taxpayer would only get the funds they have in New Zealand or on hand. So I'd guess none after airfares and living costs.
Secondly, let's assume they have the money, how will it be paid? Payment plan would end up like the user pays student loan mess, and private loans could again lead to bankruptcy. I am one of the few expats that even pays back their student loan, as most tend to ignore it or just declare bankruptcy.
Only time the taxpayer wins is if the returnees have the money, or if they intend to stay in New Zealand permanently and thus can have wages garnished, and that's a rather big if, as the welfare costs kick in to taxpayers too.