Australian Prime Minister Malcolm Turnbull arrives for his first official visit to New Zealand on Friday.
Forty New Zealanders facing deportation are being held in the Christmas Island immigration detention centre and 200 in Australian detention centres overall.
Opposition MPs want the deportations to stop and Mr Key says he doesn't think New Zealanders should be held in detention centres.
He's told TV3's Paul Henry show he'd pushed the issue with former prime minister Tony Abbott and Foreign Minister Julie Bishop, and the issue will dominate talks with Mr Turnbull at the weekend.