He said it was a possibility the leaders of United Future and the Maori Party would retain their portfolios from the previous Government, and Mr Banks may be given the corrections portfolio.
"[It's] early days for those sorts of thoughts. We need to have discussions with them, see where their interests lie. I'm not ruling that out."
Mr Key said there would be "give and take" in any agreements.
"When you form a confidence and supply agreement someone, it's like a marriage contract. You're bound by it, you're committed to it, you have to work at it to make it successful."
If Mr Banks takes the Corrections job, Judith Collins could pick up the Justice portfolio from retired MP Simon Power.
Yesterday, Mr Key told reporters that National's growth in support was "a powerful endorsement of the party, and the policies - what we campaigned on and what we stood for".
National increased its vote, a rare achievement for a Government, and it got a 5.9 per cent swing in Christchurch.
However it also lost a list MP, Paul Quinn, and could possibly lose another one, Aaron Gilmore, if the Greens pick up a seat on special votes.
If that happens, National's majority will be even tighter and a deal with the Maori Party more important.
Christchurch Central is tied between sitting Labour MP Brendon Burns and National's Nicky Wagner.
The Electoral Commission estimates special votes to be 240,247 (10.7 per cent of the total) including 19,527 from overseas, and is planning to release final results at 2pm on December 10.
The return of New Zealand First leader Winston Peters with a caucus of eight had a big impact on Saturday's vote and clearly took some votes from Labour, which fell to 27.13 per cent.
Labour lost nine seats, including list MPs Stuart Nash, Kelvin Davis, Carol Beaumont and Carmel Sepuloni.
Cunliffe and Parker jostle for leadership
Tomorrow's Labour caucus meeting is likely to be emotional, with farewells to outgoing MPs and Phil Goff announcing his plan to resign as leader. Already, leadership hopefuls David Cunliffe and David Parker are jockeying for support. Mr Goff says he will remain an MP for the full term.
On Saturday night, Act was decimated and Mr Banks, a former National minister, is now its sole MP. Last term, Act had five. Mr Dunne is again the only United Future MP and will again be Revenue Minister.
Unlike Act and United Future, the Maori Party did not pledge its support before the election. And it appears to be playing hardball. Mrs Turia is speculating its price for a deal could include a major change in Te Puni Kokiri, the Ministry of Maori Development, from a policy agency to a more operational ministry geared towards employment.
Last night, the Prime Minister said support for partial asset sales would likely be part of agreements with Act and United Future but not with the Maori Party, which opposes the plan.
Mr Key is also expected to talk to Greens co-leaders Russel Norman and Metiria Turei, but with less urgency and over a co-operation agreement rather than a confidence and supply deal.
The Parliament will have 121 MPs. The overhang of one is because the Maori Party's party vote was enough for two MPs but it retained three electorate seats.
