Prime Minister John Key says Labour is "in a bit of a meltdown" after its MP Phil Twyford, with backing from Labour leader Andrew Little launched an attack against a 1 News reporter.

Labour's campaign chairman Twyford has accused 1 News' Andrea Vance of "unprofessional" and "biased'" reporting around the costing of Labour's new jobs scheme policy.

Vance had said Labour's numbers did not add up after Labour admitted the $60 million costing for its proposed six month job scheme was based on the assumption people would take part for an average of four months.

Key said it was not the first time Labour had presented incorrect costings.

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"The Labour Party are clearly in a bit of a meltdown. They turn up, yet again, with a policy that is incorrect. I've got their policy and it says 'six months.' It is not funded that way and they can attack the media all they want as a distraction but the truth is, yet again, they got it wrong."

Asked how National would cost such a policy he said "if we are funding something for effectively four months, we say it."

The policy was the centrepiece of Little's speech at the annual conference on Sunday and provided for six months' community work on the minimum wage for young unemployed people.

The four-month calculation was not included in any of the material provided to media or mentioned by Little at the conference but the reporter, Andrea Vance, was told of it when she later questioned the numbers.

Vance referred to it in her report that night, saying "the numbers don't add up" and Labour had admitted it was costed on four months when asked about the $30 million shortfall.

After a series of tweets describing that report as "biased" and a "hatchet job" yesterday, Twyford renewed his attack this morning.

In a testy exchange with Vance as Twyford spoke to media, he said it was unprofessional journalism and accused her of "presenting as fact an assertion by Steven Joyce, the National Party spin merchant, that Labour has got its numbers wrong".

Vance replied 1 News had worked out the shortfall themselves. She said Labour had presented $60 million as the cost for a full six months by omitting to mention that it was based on four months. "You didn't get the numbers wrong, you just weren't honest about how long you costed it for."

Little also joined the fray, saying he was satisfied with Labour's handling of the issue and the policy announcement.

Asked if it was appropriate for Twyford to be attacking journalists on Twitter, he said: "I don't get to control what Labour MPs do on social media.

"I expect them to exercise a level of responsibility. I'm very thankful that we have MPs who are very passionate about Labour policies and will defend them to the hilt."

He confirmed he had known the $60 million figure was based on four months when he fronted to media after announcing the policy.

Questioned on why he had not disclosed it at ​the time, he said the media were demanding a level of detail from him that they did not expect from National.

"Some of you demand a level of detail from me that you never, ever demand from a government with thousands of public servants behind it. I stand by the policy, I stand by the numbers."

Little said Labour's concerns with 1 News' report were "being dealt with formally" but would not confirm if an official complaint had been lodged.

"I'm satisfied with the way that is being dealt with, I'm satisfied at the way we put the announcement together, the feedback and the aftermath of it."

John Gillespie, 1 News head of news and current affairs, said the network "absolutely stands by the journalism and the journalist."

"We are more than happy for Labour to follow the standard process and submit a formal complaint which will be assessed against relevant Broadcasting Standards."

Twyford also said the assumptions the party used were reasonable and were explained to Vance personally.

"But she chose not to include that information in the story. When I see journalism that is that unprofessional it has to be called out. There was no error in the numbers."

Vance's story had spelled out the cost related to a four-month period rather than six months.