Mr Key's presence could attract other party leaders - each candidate has been told the Prime Minister will be attending in case they wished to bring their leaders.
It is a recipe for fireworks, but convenor Holly Ryan said all candidates - including Mr Key - were on a "two strikes" warning.
If they sledged or even mentioned their rivals more than once, they would be ejected. "Nobody will be exempt. No political hot air."
Despite the strict rule, Ms Harre said she hoped to question Mr Key about Mr Dotcom's residency and the unlawful spying by the GCSB before the raid on his mansion.
She also planned to raise the Trans Pacific Partnership trade agreement and the SkyCity convention centre deal.
Debate between Labour's Te Tai Tokerau candidate Kelvin Davis and Mr Harawira could also be testy after Mr Davis criticised Kim Dotcom on his Facebook page and described the Internet Mana deal as "the biggest con in New Zealand's political history".
He said he made no apology for asking for donations to help thwart it and had a jibe at Mr Harawira for being thin-skinned.
"I make no apologies if there's another Maori politician in the north feeling pretty sensitive about all the criticism he's copping from hapu throughout Te Tai Tokerau because of the con job."
Mr Harawira responded by asking Labour to apologise to him for what he said was a "dirty and underhand campaign that Kelvin has made it into."
"I am asking the president of the Labour Party to issue an apology to my people in the Tai Tokerau for the dirty tricks campaign that Kelvin has been waging, and a formal declaration that it will not happen again."
Mr Cunliffe said he had spoken to Mr Davis about the need to keep attention on policy and run a positive campaign. But he said Mr Davis was simply campaigning vigorously in his electorate.
"We don't hold a candle for Kim Dotcom. Kim Dotcom has been a donor to Act candidates in the past, and has no association with Labour.
"I've made it clear there will not be Internet-Mana in a government I lead."
Mr Davis' Facebook post prompted National-aligned bloggers David Farrar and Cameron Slater to donate about $100 each toward his campaign.
Mr Davis said he would forward the money to Rape Crisis rather than accept it for himself, and thanked the two men for their contribution to his campaign against sexual violence.
The Facebook entry followed a decision by Labour's head office to veto a proposed fundraising website for Mr Davis partly because it was critical of Internet-Mana.
