National Party list MP Melissa Lee is again under investigation over the way her television production company used taxpayer funds, it was reported tonight.

TV3 News said a whistleblower sent it a letter written to Ms Lee by the broadcasting funding agency NZ on Air asking why her company, Asia Vision Ltd, did not return $100,000 of unspent contingency money.

The letter said the company seemed to have acted in a way that was in breach of its funding contract.

Ms Lee was National's candidate in the June 13 Mt Albert by-election and during the campaign faced allegations that her company had misused taxpayer funds.

NZ on Air cleared her of those allegations, and the current investigation is not connected with them.

TV3 News spoke to Ms Lee and reported: "She explained the problem by saying: `We didn't have enough money so we transferred our profit margin across to cover contingencies. We then transferred the money back at the end of the year. Technically, all unspent contingency is meant to be returned to NZ on Air. It's an accounting error in my view, I'm waiting for the outcome'."

The letter was dated June 12, the day before the by-election, but the issue was not revealed during the campaign.

Broadcasting Minister Jonathan Coleman, who is responsible for NZ on Air, said he first knew about it on August 18 and it was an operational issue that Ms Lee had to sort out.

The Labour Party, which has the same document as TV3, says Dr Coleman must have known about the letter before then.

"Unless he fronts up and tells the public what he knows, he could be accused of a cover-up," Labour's deputy leader Annette King said.

"He must have known about it. It's never been disclosed, no one has ever known it existed."

Labour intends asking questions about it in Parliament tomorrow.