MELBOURNE: Former New Zealand Prime Minister Jacinda Ardern will help tackle violent extremism online and will also be on the board of an environmental prize set up by Prince William as she looks ahead to a life after politics.

Ardern stepped down as prime minister in January saying she had "no more in the tank" to lead the country and would also not seek re-election to parliament. She is due to give her final speech in parliament on Wednesday.

Ardern, who became the youngest female leader in the world when she won power in 2017 at the age of 37, will serve as an unpaid special envoy for the Christchurch Call, an initiative she co-founded in 2019 to bring together countries and technology companies to combat extremism, the government said.

Attacks on two mosques in Christchurch, New Zealand's second-largest city, in March 2019 killed 51 people dead and wounded 40. The white supremacist gunman who carried out the assault live-streamed part of it on Facebook.

"The Christchurch Call is a foreign policy priority for the government and Jacinda Ardern is uniquely placed to keep pushing forward with the goal of eliminating violent extremist content online," her replacement as prime minister, Chris Hipkins, said in a statement.

"Terrorist and violent extremist content online is a global issue, but for many in New Zealand it is also very personal."

Ardern will also join the board of Prince William's Earthshot Prize, awarded for contributions to environmentalism, Kensington Palace said in a statement.

Ardern rode a wave of popularity dubbed "Jacindamania" after she took over as prime minister and campaigned for women's rights, and an end to child poverty and economic inequality.

She also won plaudits internationally for her handling of the COVID-19 pandemic.

But her popularity waned during her final year in power as inflation rose to nearly three-decade highs, along with rising crime and a contentious overhaul of water infrastructure.

"Five years probably felt more like nine, just because of what we all went through as a nation," Ardern said in an interview with state broadcaster TVNZ on Tuesday.

"I will miss the people ... but I won't miss the weight, because it is heavy."