WELLINGTON (BLOOMBERG) - New Zealand's main opposition party elected a new leader after a slump in opinion polls spooked its members of Parliament four months out from a general election.

National Party MPs backed agriculture spokesman Todd Muller to replace Mr Simon Bridges in a caucus vote on Friday (May 22) in Wellington.

Mr Muller challenged for the leadership after two disastrous polls this week showed support for National plummeting to as low as 29 per cent from 46 per cent three months ago.

"My focus as leader is our country's economic recovery and the strengthening of every community throughout New Zealand," Mr Muller said in a statement.

"New Zealanders need a National Government with the experience and management skills to get our country through the worst crisis since the end of the Second World War."

Mr Muller now faces the daunting task of trying to dethrone Prime Minister Jacinda Ardern, whose masterclass in crisis management during the coronavirus pandemic has seen her popularity soar.

Support for Ms Ardern's Labour Party surged to 59 per cent in a 1News/Colmar Brunton poll published on Thursday, 30 percentage points ahead of National.

Ms Nikki Kaye was elected National's deputy leader, replacing Ms Paula Bennett.

The election will be held on Sept 19.