Most of the people that China's Government wanted returned were Chinese citizens suspected of lining their personal pockets through corruption.
The issue of an extradition treaty was raised in November last year with President Xi Jinping visited New Zealand after the G20.
Mr Key said today that "whether there is progress is for another day".
The Chinese did not mention names at the time but Mr Key said they were "absolutely convinced" there were quite a few suspects living in New Zealand.
New Zealand was not working on any particular cases, Mr Key said, and getting an extradition treaty would require quite a bit of work.
Despite not having an extradition treaty, it was legally possible for New Zealand to extradite Chinese citizens, but on condition they could not be tortured or face the death penalty.