Although it could go straight into Parliament if there was unanimous agreement from MPs, Mr Key doubted that would be achieved.
Mr Seymour is the only MP currently working on such a bill and he said he had briefed Mr Key on it a few weeks ago.
He said it would be based on former Labour MP Maryan Street's member's bill, which her party withdrew last term because it was wary of such a controversial topic. Mr Seymour said he had been working on it for several months but needed time to consult medical experts and others.
He said the Act Party board had signed off on it and he had done polling in Epsom to ensure it would not endanger his hold on the electorate.
Mr Little has accused Mr Key of trying to dodge the issue by refusing a Government move on it. But he has also ruled out a change in Labour's decision not to put in a bill, saying his party's preference was for a select committee inquiry.
Mr Key said that while he doubted National would block one of its MPs putting up a bill, none had approached him wanting to do so. He said he was likely to support a bill if it struck the right protections.
"Striking the right legislative balance with clear definitions and adequately strong protections is an extremely difficult task and the fact no bill has yet passed is testament to that.
"However, Lecretia has certainly succeeded in sparking a debate on the issue."