National would be "very unwise" to push ahead with a social media ban on under-16-year-olds "at the expense of coalition unity", ACT leader David Seymour says.

NZ First leader Winston Peters agrees, calling that course of action a "seriously big mistake".

The comments came on Tuesday, after the minister in charge National's Erica Stanford told reporters she was ready to take her proposal to Cabinet and would be doing so "soon".

Stanford said she would take legislation before her colleagues for a "fulsome discussion" and see what happened.

"I'm not going to presuppose what happens at Cabinet. The prime minister will manage that," she said.

"I don't know what's going to happen. People make different decisions at select committees and Cabinet. That's the whole process."

Both ACT and New Zealand First have indicated their strong opposition to the proposal.

But National is also holding briefings with the opposition Labour Party. Between the two of them, National and Labour would have a clear majority in Parliament to pass legislation.

Seymour said National would be making a "mistake" if it ignored two of the three parties in the coalition and a third of Cabinet ministers.

"At this point, we really need to be presenting a united front," Seymour said.

"For one party to pursue a policy important to them at the expense of coalition unity would be very unwise."

Asked whether ACT would "agree to disagree" on the policy, Seymour suggested that National should be the one to do so.

"If you got two parties out of three saying no, it's not a matter of agree to disagree. It's actually the third party that agrees to disagree."

As for National's discussions with Labour, Seymour quipped: "It's not illegal, but... I wouldn't advise it."

Peters said Seymour's argument had merit, especially given the proximity of the election.

He said while National could theoretically carry on, that would be a "seriously big mistake".

"We think that common sense might prevail. Can't... the National Party see now that they're making a mistake? Surely."

 

Stanford said there had been "really good engagements" with Labour MP Reuben Davidson about her plan so far. She said other Labour MPs would also be briefed on the legislation on Thursday.

"I haven't asked Labour for their agreement. They will be able to see the bill when it's tabled. They'll have plenty of time to make up their mind," she said.

"Of course, they've made the right noises in the media... our communications so far have been productive."