New Zealand's opposition leader is at the centre of a second sexism row less than one month after taking the reins of the Labour Party.

Jacinda Ardern, 37, has almost single-handedly reignited Labour's chances at a national election in September.

But a minority party leader posted on Twitter that Ardern needs to prove she is not "lipstick on a pig", prompting another outcry following a furore when she was asked less than 24 hours after she took up the leadership role if she planned to have children.

Gareth Morgan, a prominent New Zealand businessman who founded the progressive Opportunities Party last year, drew ire on Sunday after making the comments during a Twitter conversation bemoaning Ardern as an example of the rise of politics driven by popular personalities, which he said distracted voters from policy.

Many took to social media over the weekend to accuse Morgan of misogyny.

Some New Zealand women, led by a television presenter, posted photos of themselves wielding tubes of lip gloss accompanied by the hashtagged phrase #lipstickonapig.

Morgan said in a statement on his party's website on Monday that his choice of words was not a personal attack and simply a euphemism for a "meaningless face-lift or makeover".

Ardern is the youngest person to lead the party and the second woman who has held the role.

Her rise has worried Prime Minister Bill English and led to the resignation of a minority party leader who helped the government reach a ruling majority after a 2014 election.

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Labour's support has risen about 14 percent to 37 percent since Ardern took over earlier this month, according to internal party polling leaked to media over the weekend.

The centre-left party is now just three points behind English's National Party, which has spent almost a decade in power, making a change in government a realistic possibility.

A separate TVNZ poll late last week had Ardern and English level pegging, with 30 percent support as preferred prime minister, a rise of 24 percentage points for the Labour leader.

In an interview with Radio New Zealand on Monday, English said the National was "worried" and has to "lift our support in order to form a sensible government" after September's vote.

New Zealand's complicated proportional voting system means major players have to form alliances with minor parties.

'Jacinda-mania'

Ardern's rise has been dubbed "Jacinda-mania" by local media.

Her opponents, however, criticise her as an example of politics driven by personalities and not policy.

#GarethMorgan and his hack knew full well what they were suggesting w #lipstickonapig. Old blokes don't deal w capable women well #jacinda — Dianne Davis (@DavisConsultNZ) August 20, 2017

Hey @garethmorgannz even if a phrase is commonly used, it doesn't somehow make it any less sexist 🙄🙄 #lipstickonapig pic.twitter.com/rgFuiY4iHB — Georgina (@g_townnz) August 20, 2017

Meanwhile, the National Party suffered a new blow when one of its coalition partners, Peter Dunne of the United Future Party, announced that he would not contest the election.

Recent polling showed him trailing Labour's candidate in his Wellington electorate of Ohariu.

Green Party co-leader Metiria Turei also resigned on August 9, after confessing she committed welfare fraud as a struggling young mother more than 20 years ago.