Ms Clark was previously a moderator on TV3's The Vote, and has provided political commentary for The Nation.
TV3 said she could be included on a panel for election night coverage later this year.
"We will continue to review Linda's involvement in our programmes on a case-by-case basis, taking into account that her role is clearly defined as a commentator and panellist, not a journalist, interviewer or host."
Several journalists' political connections are being scrutinised after Maori broadcaster and aspiring Labour candidate Shane Taurimu was found guilty of a conflict of interest at TVNZ.
Labour this morning
confirmed that they would not allow
Mr Taurima to seek the party nomination for Tamaki Makaurau.
The decision followed confirmation in an independent report that he had misused TVNZ funds for political purposes while he was head of Maori programming.
Mr Key said Labour's actions were expected.
But he was also surprised to learn that Ms Clark, who appears on TV3 current affairs show The Nation, was providing media training to Labour leader David Cunliffe.
"In the same way that MPs and ministers have to manage their conflicts of interest, it's also true for the media.
"I don't know how she's managing that situation but she'll just have to demonstrate to viewers that she's managing it appropriately."
Mr Key said it was not unusual for media commentators to provide these services - he was trained by former TVNZ head of news Bill Ralston.
But he said it was important that viewers knew of any political connections.
Ms Clark's law firm Kensington Swan said she was unable to discuss her clients.
Mr Cunliffe would not confirm that he was being trained by Ms Clark, saying he took advice from a wide range of people.
Mr Taurima's disqualification could open the door for another journalist, Maori Television broadcaster Julian Wilcox, to seek the Labour nomination in Tamaki Makaurau.
Mr Key said he did not believe Maori TV had been biased towards him.
But he said Mr Wilcox's potential links to Labour 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."
The investigation into political bias at TVNZ recommended that political and senior reporters should not be allowed to belong to a political party.
Asked whether he supported this, Mr Key said: "That's a tricky one. It's up to every journalist to decide how they want to handle those situations.
"In my experience, a lot of them don't vote let alone join a political party.
"It's a bit of a stretch if they're joining a political party and ... a journalist in the firing line of the daily coverage of politics, because it's hard to believe you could be that balanced in your commentary."
Mr Cunliffe said this morning that there was still goodwill towards Mr Taurima but the party had reached the right conclusion in blocking him.
"I think he needs to reflect upon... his actions and take some responsibility for them and it's a matter for him as to whether he wishes to continue to contribute to the party in future."
