National is accusing the Government of wasting taxpayers' money on using Tax Working Group chairman Sir Michael Cullen as a "hired gun," after his contract was extended.

But Cullen – who was also once a Labour Party Finance Minister – rubbished the claim and said he was only staying on to "defend the integrity of the report".

Finance Minister Grant Robertson yesterday confirmed Cullen's contract had been extended. He could be paid more than $1000 a day in the role.

He said this was pro rata, so it related to the number of hours he worked – this was a standard rate, Robertson said.

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"He certainly won't be a full day's work every day," he said. He would be paid for things such as media interviews and would not be able to charge for more than six hours of work a day.

Robertson added that Cullen was still in the role because: "there is clearly a lot of interest in the Tax Working Group's report and he needs to be in a position to respond to the questions that are asked".

In the weeks since the final report was released, Cullen had released two press statements rebuking claims that had been made about the potential impact of the proposed capital gains tax (CGT).

One was in response to National's claims about how a CGT would impact KiwiSaver, the other was about National's claims on how it would impact farmers.

National's finance spokeswoman Amy Adams accused Cullen of being "overtly political" in his statements.

"Paying a deeply political Labour Party grandee with taxpayers' dollars to engage in political debate, and effectively be a hired gun to attack the Opposition, is outrageous," she said.

Adams said Robertson and the Government had been missing in action in the CGT debate and getting Cullen to defend the report for them was "outrageous".

Act Leader David Seymour agreed.

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"For taxpayers to continue to pick up Dr Cullen's $1062-a-day fee after being appointed more than 14 months ago is outrageous."

The Government said when it received the report in early February that it would provide a full response in April.

Speaking to the Herald, Cullen shrugged off Adams' accusations of partisanship.

He said he had no particular definition of his role now, but it does include "defending the integrity of the report against people misrepresenting it".

This included defending the Tax Working Group's processes, its conclusions and its work as a whole.

"It is unfortunate that a number of people seem to have difficulty actually describing the report in terms of what it actually said."

The National Party has been in overdrive with its criticism of the potential impacts of the CGT Cullen recommended.

He said the misrepresentation was not just from National and it had, in fact, been fairly widespread.

"There is so much misrepresentation going on I'm very pleased my contract is being extended."

He rejected Adams claims that his contract being extended was an "overtly political move".

Cullen said it was his role to point out if people or groups are misrepresenting what the group said.

His contract has been extended until June 30 – "basically for some time beyond when the Government is intending to make its decisions".

This means he would be getting paid for at least two months after the Government had released its full recommendations.

"Once the Government has made its decision, I see my role as no longer relevant," he said and added he had turned down an invitation to speak in August.

Asked if Cullen was being politically neutral, Robertson said he was "responding to the issues that have been raised".

"He is responsible himself for the tone of those comments."