By Gyles Beckford
WELLINGTON (Reuters) - New Zealand Prime Minister John Key has shrugged off a lingering 'dirty tricks' political scandal, and his center-right National Party is on course for a big victory in this month's general election, opinion polls show.
Three new polls showed support for Key's party softening slightly, despite the scandal that has claimed a senior minister, but still well ahead of the main opposition center-left Labour Party.
A Reuters poll of the five main surveys put National's support at 48.2 percent, its lowest level in six months but still above its mark set in 2011 election. The Labour Party's support stands at 26 percent.
New Zealand goes to the polls on Sept. 20.
National is campaigning on its record of strong economic growth, a return to budget surpluses and paying back debt, while it promises modest new social spending, as well as the prospect of future tax cuts.
The latest batch of surveys came as Key set up an inquiry into allegations that a senior minister was involved in a campaign to undermine a senior law official.
The Justice Minister Judith Collins resigned last weekend amid a swirling controversy linking her and officials in Key's office with a right wing commentator, who was given personal and secret government information to use against government critics and opponents.
Collins denies the allegations.
The inquiry, likely to be headed by a retired judge, was not expected to report back until after the vote.
"The matters to be investigated in relation to Ms Collins are serious, and I believe it is important the inquiry have sufficient time to conduct a thorough review of those matters before reporting back," Key said.
Key's personal popularity and that of his party have remained largely unaffected by the controversies, with their support often above 50 percent as the economy has emerged strongly from recession and the aftermath of two devastating earthquakes.
An analyst said it was a surprise that Key and his party had not suffered more in the polls, and were still favorites to form the next government.
"Key has had a strong strategy of stonewalling on the issue and it looks like it's worked, and he's convincing people that there's not a lot to see here," said Otago University political scientist Bryce Edwards.
Under New Zealand's proportional voting system no party has ever gained an outright majority, with the biggest party reliant on minor parties to govern.
National has been supported over the past six years by the free-market ACT Party, the centrist United Party, and the indigenous Maori Party.
The final shape of the government might be determined by the economic nationalist New Zealand First Party, led by the veteran politician Winston Peters, who in the past has variously supported National and Labour.
Key has not ruled out working with Peters, but said he would wait for the election result before considering any approach.
(Editing by Jeremy Laurence)