At some point, either New Zealanders will decide to borrow less or foreigners will decide to lend us less. If foreigners tire of lending us their savings before we New Zealanders tire of borrowing them, the likely outcome is a change in the mix of monetary conditions, in the direction of a lower exchange rate and somewhat higher interest rates.
We have ample experience of how things can go wrong, for example, the Asian crisis in the late 1990's, and the series of crises in Latin America around the turn of the millennium. And, also Finland has first hand experience from a severe banking crisis that took place less than 20 years ago. There are many lessons to be drawn from these crises.