A law making marriage under the age of 18 illegal is expected to be soon tabled and passed in Papua New Guinea's parliament.

There is currently no legislation in PNG to regulate the marriage age, meaning child marriage is practiced legally.

PNG Constitutional Law Reform Commission secretary Eric Kwa said the issue of child brides is "spread throughout the country".

UNICEF's 2015 statistics found 2 per cent of women aged 20-24 were married by the age of 15, with 21 per cent married by 18 years of age.

"We are aware and we are mindful that this is happening," Mr Kwa told Pacific Beat.

"So we need to address this at a national level."

Mr Kwa said PNG's cabinet had signalled its intention to approve the law.

The issue, he said, would be enforcing the legislation, especially in the "far-flung areas of the country".

"We know it's going to be a major issue for us in terms of implementation, but we would like to enact the law and then inform the people," he said.

The law is being introduced alongside a raft of other legislation designed to protect children in PNG, including revisions to the country's Lukautim Pikinini Child Protection Act.

Mr Kwa said the marriage laws will "gel together" with recently-introduced free education and free basic healthcare legislation.

'So long as they're menstruating, they're ready to be married off'

Women's groups in the Pacific said they will be watching the PNG law to see how it could be reflected in the region.

In Vanuatu, the law forbids marriage of women before the age of 16, and for men before the age of 18.

But Vanuatu Women's Centre (VWC) coordinator Marilyn Tahi said throughout Melanesia, traditional perspectives were still prevalent.

"Our people even have traditions where so long as they're menstruating, they're ready to be married off," she said.

"Others say, so long as they have breasts, they're ready to be married off."

Ms Tahi called the inconsistency between the legal age of marriage for men and women in her country "unfair" and said the VWC had been pushing to bring the age up to 18.

She said the revisions to PNG's child protection act could spark similar changes throughout the region, including in Vanuatu.

When Vanuatu was the first nation in Melanesia to criminalise domestic violence in 2008, "a lot of the other Pacific countries got on board".

"We can learn from each other," she said.