Judith Collins is questioning whether Winston Peters was just playing the National Party during coalition negotiations in light of the legal action he has taken against National MPs - filed the day before the election.
Peters, who is Deputy Prime Minister and New Zealand First leader, is seeking information relating to his superannuation payment details, which were leaked during the election campaign.
This week he served legal papers on several people including National leader Bill English, English's then-chief of staff Wayne Eagleson, deputy leader Paula Bennett, former ministers Anne Tolley and Steven Joyce, the head of the Social Development Ministry Brendan Boyle, and two journalists.
Collins questioned whether Peters could have sincerely considered a coalition deal with National, when he filed the legal papers almost a month before he announced a coalition deal with the Labour Party.
"At the time, we were very much convinced on our side there were genuine negotiations going on," she told Newshub's Am Show.
"But I've got to say, it's not looking like it was quite so genuine anymore. I think Mr Peters should really explain himself to the public because certainly there are a lot of voters who were disappointed in his decision.
"I think New Zealanders are owed an explanation. Was he being genuine, or was it just a play?"
It is the first time a National MP has openly questioned Peters' integrity during the coalition talks.
It was revealed in August that Peters had repaid thousands of dollars due to years of overpayment in his superannuation. It is unclear how the mistake happened.
Peters' legal action is seeking information to identify those responsible for leaking the payment details to the media. Newsroom's Tim Murphy and Newshub's Lloyd Burr are also named in Peters' documents.
Peters can ultimately seek damages if he can identify who was responsible for breaching his privacy.
Labour's senior MP Phil Twyford defended Peters.
"Every other bit of information we have suggests that he played it right down the line with the coalition negotiations.
"He negotiated in good faith with both parties. It went right down to the wire. The fact that it didn't go to the way of the National Party doesn't mean it's fair game to reopen the whole thing now.
"If the Deputy Prime Minister feels that information that belongs to him was illegally leaked, then he's perfectly entitled to explore his legal options, but you can't put that on the same level as the decision to choose a coalition partner."
The legal action has prompted concerns about the rights of journalists to protect sources - though journalists can seek to withhold information that might disclose the identity of a source under section 68 of the Evidence Act 2006.