Last month, Charmaine Backhouse spent Christmas with her son Isaiah for the first time in five years.

Key points: Canberra was unable to assist as PNG is not a signatory to an international abduction convention

Canberra was unable to assist as PNG is not a signatory to an international abduction convention Ms Backhouse researched PNG legislation that she then used to regain custody

Ms Backhouse researched PNG legislation that she then used to regain custody Father previously said it "wasn't entirely true" that he refused to let Isaiah travel to Australia

"It was happy tears all around," she said.

"He was so excited and it was a lovely day and I'm so grateful for it."

A few weeks earlier, Ms Backhouse had travelled to Papua New Guinea and regained custody of Isaiah from his Papua New Guinean father.

It was the culmination of a long battle to bring the boy back to Australia, one that Ms Backhouse fought largely on her own without much help from Australian authorities.

"I never thought about not getting him back," Ms Backhouse said.

"I was just determined".

Son's visa expired in 2016

Until last month, Isaiah had been living in PNG's capital Port Moresby with his father, Dr Duncan Dobunaba.

The now nine-year-old went there in 2015 when his parents' relationship ended.

According to Ms Backhouse, she and Dr Dobunaba mutually agreed at the time that Isaiah would go to PNG and temporarily live with his father while she moved from Brisbane to Broome in Western Australia.

Isaiah was born in Brisbane and is an Australian citizen. ( Supplied )

But since then, she claims, Dr Dobunaba refused to send him back to Australia even though Isaiah is an Australian citizen who entered PNG with an Australian passport on a visa that expired in April 2016.

In November last year, when the ABC first reported on Ms Backhouse's efforts to regain custody of her son, Dr Dobunaba said via text message "there are always two sides to a story" and "it is not entirely true" that he refused to allow Isaiah to travel to Australia.

He declined to comment further.

PNG urged to sign convention on international abduction

Ms Backhouse initially approached the Australian Attorney-General's office for help to bring Isaiah back but was told there was little the Government could do because PNG is not a signatory to the Hague Convention on the Civil Aspects of International Child Abduction.

The convention provides a process for a child to be returned to their country of "habitual residence" for custody issues to be dealt with.

States in light blue have signed and ratified the convention. States in dark blue acceded to it. ( Wikimedia Commons )

So instead she searched for an avenue in PNG to regain custody. She came across the country's child welfare legislation, called the Lukautim Pikinini Act.

Simon Yanis, an official with the Office for Child and Family Services, said the case was the first time a foreigner used the legislation to gain custody of a child in PNG.

"I googled a lot of it," Ms Backhouse said.

"I searched up anything to do with family law, to tell you the truth, and then I came across that link and I read it thoroughly over and over again."

The law, which had been revised in 2015, established the Office for Child and Family Services and PNG's Family Court, which make determinations on child custody matters.

Ms Backhouse contacted the office and early last December it helped her to successfully apply to PNG's Family Court for custody of Isaiah and permission to bring him back to Australia.

"I am very grateful for how they assisted me," Ms Backhouse said.

"If that failed I would've found some other avenue and would've continued to keep fighting to get my son back."

Despite her success, Ms Backhouse hopes the PNG Government signs up to the Hague Convention so that in future there's a smoother process for parents who find themselves in similar circumstances.

"It's cost me a lot, my emotions, my mentality, it's hurt my pocket big time. I've had to work overtime just to get the money to get over there," she said.