MMP allows parties which win an electorate seat to bring in other MPs even if they do not reach 5 per cent of the party vote.
Prime Minister John Key said Mr Cunliffe would try to form a government with the Internet Mana which had a similar deal and Labour had tried similar deals with Alliance and Green MPs in the past.
"A little bit of consistency would be good." He believed voters knew MMP well enough to make the choices they considered best.
Mr Cunliffe said he had made it clear it was "extremely unlikely" any Internet Mana Party MPs would get ministerial positions, or even lower level associate or undersecretary roles in a Labour-led Government.
But he would not rule out policy concessions in return for their votes, saying that was a matter to discuss after the election. "We will talk to whoever the voters serve up."
Mr Key said he doubted Labour would not include Internet Mana in Cabinet if it was needed to form a government.
"The reality is David Cunliffe about 10 months ago came into the job of Leader of the Opposition and said he was going to deliver a result in the high 30s for Labour and that would see them as the next government. Then he downsized that to the low 30s. In recent times, he's been saying Labour in the 20s could still theoretically become the government. What we know is when you're Leader of the Opposition you're desperate to become Prime Minister and will probably do anything. He's in the camp of forming an alliance with anybody to get over the line."
Mr Cunliffe denied it was a double standard.
"Because I'm not tryng to tell New Zealanders who to vote for. I'm being absolutely plain that they should vote Labour with two ticks."
Mr Cunliffe said Labour was not doing any similar deals itself and would abolish coat-tailing if in Government.
He said it was no different to Key saying National could work with the Conservative Party.
When it was pointed out that National's refusal to do a deal in East Coast Bays meant the Conservative Party would have to reach five per cent of the party vote to get in, Mr Cunliffe replied: "So will Internet Mana when [Labour's candidate] Kelvin Davis wins Te Tai Tokerau."
He said Mr Key seemed to think that being open about the deals meant it was not a manipulation of MMP, and he did not agree with that.