but if there's no shortage why has one New Zealand turbo prop operator reported to have been offering visa sponsorships?
Economics 101: supply vs demand. As supply is increased, the price will fall. Q.E.D if you want to prevent wage increases, increase supply. Two ways to do that, train them, or import them.
Welcome to Globalization of wages. It isn't about boosting developing world wages, rather pressuring the developed world down to the third world standard.
Citizenship [or the prospect of it] & residency are the carrots to facilitate this process. NZ has been a poster child for this method of suppressing wages, AU isn't far behind.