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But Sir Michael, a former 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 confirmed this week Sir Michael's contract had been extended. He could be paid more than $1000 a day in the role.
Mr Robertson said this was pro rata, so it related to the number of hours he worked.
"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.
Mr Robertson said Sir Michael 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, Sir Michael had released two press statements rebutting claims that had been made about the potential effect of the proposed capital gains tax.
National finance spokeswoman Amy Adams accused Sir Michael 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.
Ms Adams said Mr Robertson and the Government had been missing in action in the CGT debate and getting Sir Michael to defend the report for them was "outrageous''.
Sir Michael shrugged off Ms Adams' accusations of partisanship. He said he had no particular definition of his role now, but it did include "defending the integrity of the report against people misrepresenting it''.