However, he stood by his earlier statements that casinos were generally safer gambling environments than pubs and clubs.
"My own view is that they are well controlled environments and what concerns me about pokie machines being spread across the community and wider suburbs is that if you look at a density map of those pokie machines, they typically prey on low socio-economic groups. There's a high proliferation of those machines in West Auckland and South Auckland.
"At least with a casino it's a much more determined action to go there."
Mr Key also said evidence as to whether casinos encouraged more problem gambling than pubs and clubs was mixed. "I'm not convinced that a modest increase in pokie machines at a casino would have a significant impact on problem gambling in New Zealand."
While dismissing reports of up to 500 more machines as speculation, he refused to indicate what the number may be other than to say it was "a small adjustment up".
"But the overall number of pokie machines that Auckland has will plateau and fall over time."
Mr Key has previously said the Auckland Council's "sinking lid" policy on gaming machines in pubs and clubs means fewer gaming machines overall.
Meanwhile, should the deal reach fruition, any changes to the Gambling Act that would be required would not be conscience vote for National MPs.
That was because Mr Key regarded the issue as primarily an economic one.
"It's largely the issue of a piece of infrastructure for tourism and it's an important part of building that tourism model."