He said most New Zealanders would vote National or Labour and it made sense for viewers to get the chance to consider their potential prime ministers.
"The leaders' debate is an interesting opportunity for New Zealanders to get a sense of who they want to see as the next Prime Minister of New Zealand and practically, at the moment, the only two contenders for that job are Phil Goff and myself."
He did not accept that undermined MMP and said his Government had worked well with the Maori, ACT and United Future parties. A separate debate for those parties was good for them, he said.
Mr Goff said he would not appear in debates where Mr Key did not.
"If he's not going to be there, then that takes away the main debate between the two major parties. He should be there," he said.
Mr Goff said there would be a lot of opportunities for election debates and he had committed to five or six.
"Interestingly to date, John Key has not engaged in any direct debate, he's turned them down."
Mr Goff said he did not know if his staff had discussed the issue with Mr Key's office.
"I've made the point quite clear -- on any platform that John Key is going to be up there, I'm ready to debate with him over issues that really matter to New Zealanders."
In 2005 there was a row when TV3 tried to exclude United Future leader Peter Dunne and Progressive Party leader Jim Anderton from a debate on MMP.
The network said eight leaders was too many, but Mr Dunne and Mr Anderton went to court and TV3 had to let them take part.
- NZPA