New Zealand Prime Minister Jacinda Ardern and her long-time partner are engaged to be married.

Ardern, 38, and television show host Clarke Gayford, 42, became engaged over the Easter break, a spokesman for her office confirmed on Friday.

They declined to comment further.

Ms Ardern and Mr Gayford met in 2012. Their first child, Neve, was born in June last year – only the second child to be born to a serving national leader.

Quizzed in a BBC interview earlier this year about marrying Mr Gayford, Ms Ardern said she would not propose to him.

“Absolutely I am a feminist’, she said.

“But no, I want to put him through the pain and torture of having to agonise about that question himself. No, that’s letting him off the hook.”

The prime minister’s pregnancy made her a role model as only the second-ever elected female leader to have a child while in office – after Pakistan’s Benazir Bhutto in 1990 – although she played the milestone down, saying she was not the “first woman to multi-task”.

A former small-town Mormon, Ms Ardern’s meteoric rise from deputy of a flailing opposition Labour party to the top job in just months was dubbed “Jacindamania” and made her a rock-star of the global political left.

In recent weeks she won widespread praise for her compassionate approach in the aftermath of the Christchurch mosque shootings on March 15.

News of the engagement emerged as Ms Ardern joined families of the 29 men killed in a November 2010 mine disaster for a ceremony at the site’s entrance on Friday.

The long-awaited re-entry to the Pike River Mine on the west coast of the South Island was delayed at the last minute on Thursday by an unexplained spike in oxygen within the mine.

It was another setback for families who have for years fought to have the mine re-entered in hopes of discovering what caused the explosions and finding the remains of their loved ones.

Jacinda Ardern and Joanne Ufer in Greymouth on Friday. Photo: AAP

Joshua Ufer, 25, was one of two Australians killed when methane blasts tore through the mine.

His mother, Joanne, and seven-year-old daughter, Erika, walked with Ms Ardern as relatives and supporters arrived at the entrance on Friday.

“It’s where we needed to be. With all the other families who have travelled this road,” Ms Ufer said.

Despite the cancellation, organisers opted to proceed with an event where the re-entry would have begun because relatives had already gathered in nearby Greymouth.

“It’s not happening today but it’s going to happen very, very soon,” Sonya Rockhouse said in an address on behalf of the families.

“My youngest son, Ben, who was only 21, is still in there somewhere. I promised I would bring him home every day and from today we are starting to fulfil that promise.”

Mr Ufer’s daughter – who carried an Australian flag on Friday – was born in Greymouth six months after the blast. His mother said it was important to bring the family together to see the site on what she said had been an emotional day.

Authorities say they’re still determined to re-enter the mine but aren’t sure when.

William Joynson, 49, was the other Australian killed.

-with AAP