Judith Collins says the post-election negotiations between her party and Winston Peters appear to have been a fraud.
It was revealed on Thursday the New Zealand First leader's legal action against journalists, the head of the Ministry of Social Development, a number of National MPs and their staff was filed the day before the General Election, which was held on September 23.
On October 19, 12 days after the election results were confirmed, Mr Peters announced he'd come to an agreement with Labour and the Greens to form the next Government.
Ms Collins told The AM Show on Friday morning it now appears Mr Peters was playing the National Party, and never intended to sign a coalition agreement with them.
"At the time, we were very much convinced on our side there were genuine negotiations going on. But I've got to say, it's not looking like it was quite so genuine anymore."
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Details about Mr Peters' superannuation overpayments were leaked to the media in August. His legal action aims to "identify and sue" those responsible. The National MPs targeted are Bill English, Steven Joyce, Paula Bennett and Anne Tolley. The journalists named in Mr Peters' documents include Newshub's Lloyd Burr.
"I think Winston Peters should really explain himself to the public because there were a lot of voters who were disappointed in his decision," said Ms Collins.
"I think New Zealanders are owed an explanation. Was he being genuine, or was it just a play?"
Election statistics released in late October show 41,400 people who gave their party vote to NZ First ticked the National candidate, but marginally more - 44,600 - gave theirs to the Labour candidate.
Labour's Phil Twyford, also appearing on The AM Show, said Ms Collins couldn't assume Mr Peters would never have signed an agreement with National, despite taking legal action against four of their highest-ranked MPs.
"He negotiated in good faith with both parties. It went right down to the wire. The fact it didn't go the way of the National Party doesn't mean it's fair game to reopen the whole thing now."
He said Mr Peters is "perfectly entitled to explore his legal options" over the leak.
The AM Show has extended an invitation to Mr Peters to appear on the show to give his side of the story.