TV3 did not confirm or deny that Ms Clark worked for Mr Cunliffe, and her law firm Kensington Swan said she could not discuss her clients.
Mr Cunliffe refused to discuss whether he had worked with her.
Prime Minister John Key said it was not unusual for political commentators to work for parties, but he was surprised she was working for Labour and said she would have to manage any potential conflicts of interest.
Mr Key was trained by Bill Ralston, who is the former head of news at TVNZ.
Ms Clark's work for Labour came into the spotlight at Parliament yesterday after another broadcaster, Shane Taurima, was barred from running for Labour because he had campaigned for the party while still at TVNZ.
Party president Moira Coatsworth said the party still believed Mr Taurima had a political future, but it was not in its interest for him to be the candidate in Tamaki Makaurau at the general election.
That role could be taken by another broadcaster, Maori Television's Julian Wilcox, whose political leanings were also raised in Parliament yesterday.
Mr Key said he had never felt Maori TV had been biased towards him, but the rumour that Mr Wilcox could run for Labour could affect the channel's appearance of impartiality.
"It could raise some issues in the public's mind. They're interviewing me, they're interviewing others, and it's the perception of the conflict of interest ..."
Ms Coatsworth refused to confirm whether she had met with Mr Wilcox about running for the Maori seat in Auckland.
The refusal by the Labour Party Council to give Mr Taurima a waiver to become a Labour candidate upset some people within the party.
The Tamaki Makaurau electorate council urged the party to review the decision.
