Opposition parties say they are readying themselves for an early election amid new speculation John Key will go to the polls in September or early October.

Sources say Key wants to hold the election before November's G20 leaders' meeting in Brisbane and Apec Summit in Beijing.

He will also want to avoid clashing with home All Black games in August and early September, the final Bledisloe Cup test on October 18, and Labour Weekend, the final weekend of October.

Pundits are betting on September 27 or one of the first two Saturdays in October.

Yesterday, Labour leader David Cunliffe said: "I wouldn't rule out that National will go considerably earlier than that if they feel desperate. This is an election that we are ready to fight.

"Nothing is surer than the fact that John Key wants to strut the world stage at the G20, and he can't do that under the shadow of an election campaign," he said.

"I'm expecting it will be my first G20 invite, and I'm expecting the election will be before that."

Labour trails National in the polls, but has a probable coalition partner in the Greens.

Cunliffe said he would be happy to be sworn in as a Labour prime minister with fewer seats than National, relying instead on the support of the Greens.

"The prime minister should be the leader of the governing coalition. The governing coalition should have the most seats in Parliament. That is the constitutional position and the proper one."

Kim Dotcom would not speculate on an election date but said "We will be ready."

(Source NZ Herald)