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He told his weekly post-Cabinet press conference that it would be preferable not to leave the country to attend the high-powered meeting of the world's most powerful 20 economies, which Australia has invited New Zealand to attend this year.
Describing the G-20 as a "complicating factor", Key dodged questions about exactly when the election could be, but was lukewarm on attending an international meeting during the course of an election campaign.
"You can do anything you want to do, but I just don't think that's likely," he said. Asked whether that meant an election would be before the Nov. 15 meeting, Key replied: "Or after."
However, the situation is all the more complicated by the fact that Key has invited several of the world leaders heading to Brisbane to make flying visits to New Zealand around the time of the G-20, including US President Barack Obama.
If they arrive before election day, their presence could be criticised as designed to influence the election, while also requiring much of the Prime Minister's and other senior politicians' campaigning time.
If held after the G-20 meeting, a four to six week election campaign would push polling day into Christmas or beyond.
Under the Electoral Act, the latest date for the election is around mid-January, but conventional wisdom dictates that is a terrible time to hold a general election. Voters are likely to be disengaged or antagonised by an election at that time and too many voters would be out of their normal electorates on holiday, which would require them to cast special votes, and would likely affect voter turnout.
Key said he would announce the election sooner rather than later, reasoning there was little political advantage in announcing the date late.
"I don't know exactly when it will be," he said. "I know a few dates that might work, but I don't have an exact date."
Responses from world leaders to the invitations to visit are likely to be deciding factors. Even attending the G-20 immediately after a general election could be difficult.
If there was a "complicated" outcome, requiring coalition-building and a two week minimum wait for the final count of special votes, there might not be clarity as to who should attend, Key or the only other serious contender, Labour leader David Cunliffe.