National MP Melissa Lee had told NZ On Air she believed her company was entitled to money she is now being told to pay back.

Ms Lee, a list MP, was National's candidate in the June 13 Mt Albert by-election and during the campaign faced allegations that her television production company Asia Vision Ltd had misused taxpayer funds for political purposes.

NZ On Air cleared her of those allegations but in the process uncovered $80,000 of unspent contingency funds that should have been refunded.

Prime Minister John Key accepts an accounting error was to blame for MP Melissa Lee's television production company needing to pay back taxpayer money, but Labour say there has been a cover-up.

Last night it was revealed that Ms Lee had been asked by funding body NZ On Air to explain why her company, Asia Vision Ltd, failed to return up to $100,000 of unspent contingency funds.

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

Broadcasting Minister Jonathan Coleman will face questions in Parliament today as Labour tries to find out when he first knew a National Party MP's company was under investigation.

TV3 News reported last night list MP Melissa Lee had been asked by NZ on Air to explain why her television production company, Asia Vision Ltd, failed to return $100,000 of unspent contingency funds.

The letter from NZ on Air, which provides taxpayer funding for locally produced programmes, said the company seemed to have acted in a way that was in breach of its funding contract.

The showdown date for Helen Clark’s vacated Mt Albert seat has been set by prime minister John Key, Saturday June 13.

The by-election date has been a source of speculation since Helen Clark was appointed as the new head of the United Nations' Development Programme, forcing her resignation.

Helen Clark’s soon-to-be vacated Mt Albert electoral seat is currently the subject of internal strife for the party, as seemingly every possible candidate has put their name forward for selection - while National mulls over its options.

Phil Twyford has long been mooted as Miss Clark’s replacement, but his election to the Mt Albert seat would see his list seat turn over to Judith Tizard – a prospect Labour has whipped itself into a frenzy over.