Sir Michael Cullen is assuring taxpayers he expects to invoice for no more than two days this month for his fee of $1000 for a six-hour day.
The government has come under fire from the National Party after it was revealed Sir Michael as tax working group chair was still being paid even after the release of the group's report.
By comparison, Victoria University emeritus professor Bob Buckle, who chaired the 2010 Tax Working Group set up by the then-National government, did the job for free.
He did not receive a cent for his work - either for the hours he put in analysing the tax system or for the time spent explaining the group's report after it was released.
"I wasn't paid anything extra to do this. This was something I agreed to do," he said.
The row over Sir Michael's pay flared in Parliament yesterday, with National's Amy Adams grilling Finance Minister Grant Robertson.
"Why is Sir Michael Cullen to be paid more than $1000 a day of taxpayer funds to engage in political debate for four months after the Tax Working Group has been disbanded?" she asked him.
The opposition was also irked by some of the comments Sir Michael has made in recent days, including when he told Morning Report on Wednesday that National leader Simon Bridges was being "hysterical" with his "destroy-New Zealand-way-of-life reaction to the report".
Speaking to RNZ yesterday, he denied he had been acting as a mouthpiece for the government.
"The major purpose is to point out where there are errors of fact or interpretation and, indeed, those are very widespread - it's by no means confined to National Party spokespeople.
"There've been a very large number of statements put in the public arena which are lacking in accuracy and often misrepresent what the report actually says and in some very important instances."
He made no apology for continuing to charge the government for his services, but said for the entire month of February - when the Tax Working Group had all but finished its report - he invoiced the government for only two days work and expected no more than that for March.
Sir Michael can continue to bill the government until June.
The government is expected to release its response to the Tax Working Group's recommendations, which include the introduction of a capital gains tax, next month.