In setting September 20 as the date for the election, the Prime Minister has just managed to avoid looking like he is using his position to questionable advantage when it comes to boosting National's chances of securing a third term on the Government benches in Parliament.

John Key has brought the election forward by two months from its traditional late November timing - about as much as he could manage to National's advantage without looking like he was putting his party's interests ahead of everything else in order to nullify negatives such as expected interest rate rises.

Given National's current high ratings in most polls, Key and his colleagues will have considered dissolving Parliament and holding the election even earlier. But that would have risked upsetting voters who would have felt they were being manipulated - and might have rebelled at the ballot box accordingly.

Key's rationale for bringing the election forward is based on two reasons. It is certainly sensible to allow plenty of time for post-election negotiations so that the person who is going to be prime minister for the next three years can accept Australia's invitation to attend the summit of the world's G20 group of large economies which the Abbott Government is hosting in Brisbane in late November.

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However, it is a bit naughty to use Winston Peters as an excuse for making the gap between election day and the G20 meeting (along with an associated odds-on brief stopover in New Zealand by Barack Obama) close to two months on the basis - according to Key - that negotiations with the New Zealand First leader, if necessary, on the post-election formation of a Government could take that long.

Knowing the date is of some benefit to Opposition parties, however, in terms of planning, fund-raising and being able to spend money on advertising without risk of breaching electoral law.

Labour's David Cunliffe was putting on a brave face yesterday following Key's announcement and conceding nothing. But Key has cut the time that Labour and the Greens have to project a Government-in-waiting from eight months to just over six months. Or five months if you consider such a perception will have to be pretty well-established in voters' minds by the kick-off of the four-week election campaign proper. Cunliffe insists Labour is ready. The Greens may be. But Labour does not look it - yet.