Steven Joyce says he would have advised against leaking Winston Peters' super details

National's campaign manager and MP Steven Joyce has publicly denied leaking details of Winston Peters' superannuation overpayment and says if anyone in National had approached him about it, he would have told them not to.

"We didn't leak Winston's superannuation details and to this day, I do not know who did," he told a Victoria University symposium on the 2017 election.

"I wasn't aware of it until it was in the media. It didn't help us and, frankly, it was never going to," he said.

"Winston is a wily politician. He will use everything to his advantage. If somebody had come to me, and nobody did, and says 'look I've got this really brilliant idea' I would have told them to go and flush it down the toilet because it actually wouldn't have achieved anything. It would have just given him more oxygen."

Joyce also denied Peters' claims that National "went after" New Zealand First in the election campaign.

Joyce said National had not expected candidate Matt King to win back Northland from Peters, the New Zealand First leader, who won in a March 2016 byelection.

He said Labour voters had helped Peters to lose that seat because its candidate, Willow-Jean Prime, campaigned vigorously and won about 8500 votes, when she won only about 1500 in the byelection.

Joyce said he does believe that either issue, superannuation or the loss in Northland, influenced New Zealand First's final decision to back Labour to lead the Government, after running parallel negotiations.

"It may surprise you but in this instance I believe Winston when he said on the night of the announcement of the appointment of Labour to be the Government that he had a choice of modified status quo or major change and he wanted to go with major change.

"I think it is as complex and simple as that."

Speaking at the same symposium, Peters, who is now Deputy Prime Minister in the new coalition Government, said his party entered negotiations with National and Labour in good faith.

"We also believe that that good faith was reciprocated."

New Zealand First wanted a coalition partner, a political party that would agree to a shared policy vision going forward.

"We knew we couldn't win everything but we wanted to work with a party that had committed to a shared policy vision and would work with New Zealand First to make change happen."

There had to be a human face behind every policy choice and commitment. That view had been lost in the past 30 years, Peters said.

"When wealth becomes too grossly unequal, our view is that people are then pitted against each other and that is unacceptable. It comes at enormous societal cost…"

A clear majority of New Zealander had voted against the status quo on election day.

Peters has filed legal action alleging a breach of privacy over the superannuation overpayment story which broke during the election campaign.

Prime Minister Jacinda Ardern told the conference that if the numbers had stayed the same between the provisional results and final results, she would not have formed a Government.

In the provisional result, Labour, New Zealand First and the Greens combined had just 61 seats between them, a bare majority in a 120-seat Parliament.

After the final results came in, National lost two and Labour and the Greens each gained a seat, giving the three-party bloc 63 seats.