Labour's campaign manager Phil Twyford has taken to Twitter to criticise media and defend Labour's policy costings..

Labour has released detailed costings of its plan to offer long-term unemployed youth six months paid work, after the Government questioned the accuracy of its $60m estimate.

It followed the party's campaign manager Phil Twyford getting into a public row on Twitter after he accused some media of bias, a lapse of professional standards and a "hatchet job" on the policy.

He said the "numbers do add up".

Not so. Andrea was briefed personally on modelling and assumptions. There was no mistake.Numbers do add up. @CTrevettNZH @avancenz @1NewsNZ — Phil Twyford (@PhilTwyford) November 6, 2016

The party has also lodged a formal complaint with TVNZ over its coverage.

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Labour is particularly sensitive to coverage that could undermine its reputation for financial management, which it sees as crucial to its chances in next year's election.

The policy was unveiled in leader Andrew Little's speech to Labour's centenary annual conference in Auckland on Sunday.

​He said the "Ready for Work" policy would offer all young people who had been on the Jobseeker Allowance for six months, full-time employment for six months at the minimum wage of $15.25.

Labour has assumed 10,000 would be involved each year, and it costed the policy at $60 million a year.

But in response to media queries, Labour has since disclosed that was based on an average time on the scheme of four months, not the full six months.

Tertiary Education Skills and Employment Minister Steven Joyce said Labour had badly under-costed the policy.

But Little said the assumptions were based on similar programmes in New Zealand and overseas, and he stood by them, though if New Zealanders did not follow the overseas trend the costs could rise.

On Monday, Little's office released details of how it had costed the scheme, in a move to rebut the criticisms.

They showed the net after-tax cost of employing someone on the minimum wage for six months, rather than paying them the benefit, would be about $8700.

The available data from Work and Income suggested that over the course of a year there are about 12,500 young people who have been receiving Job Seeker benefits for six months or more - from which Labour assumed 10,000 would be covered by its scheme, because the rest would likely find a job or go into employment, training or formal education.

That would give an-all up cost of $87m a year, but Labour said that would be too simplistic and incorrect because not everyone who started on the Ready for Work programme would finish it.

It had assumed, on the basis of evidence of how long people stayed on Jobseeker benefits, ​that on average those on the scheme would spend four months each on it. While most would complete the full six months, many would not.

After adding $3m for the net costs to the Ministry of Social Development it came to a final figure of $59m a year - which it had rounded to $60m.