Kiwi backpacker Christina Annesley's mysterious Thailand death to be raised with British PM Theresa May

Christina Annesley's body was found on the island of Koh Tao. Photo / via Facebook

The family of a New Zealand-born woman found dead on a Thai island is fighting for better consular assistance for relatives of those who die overseas.

Christina Annesley died on Koh Tao Island three years ago, after apparently taking antibiotics for a chest infection.

The Londoner, who was 23 when she died, was born in Lower Hutt.

Christina's father Boyne said he doubts he and his wife Margaret will ever find out how his 23-year-old daughter died. Photo / Supplied

Though she had spent most of her life in the UK, many of her family members were still based in New Zealand.

Her death is still unexplained - her body was left in a temple for days before a post-mortem examination was performed and the UK coroner her family contacted upon her arrival back home would not accept the results.

However, Boyne Annesley and his wife Margaret wanted new rules to help families who find themselves in the same position.

Christina Marian Annesley was found dead in January 2015, in a bungalow on the Thai island of Koh Tao. Photo / Facebook

The Annesleys were among a group delivering a petition to British Prime Minister Theresa May tomorrow, calling for better translation services, as well as for warnings to be issued about countries where multiple young Britons had died.

"It's too late for us, we don't think we'll ever be able to find out what happened to our daughter," Boyne Annesley said.

"The petition we're doing is to get the Foreign Office to advise people going to Thailand that the country is dangerous.

"If the worst does happen, don't allow any examination over there and get them back to the home country as soon as possible."

Boyne Annesley said the petition had come about through discussions with other families in similar positions.

"It was the mothers that were beginning to contact and query, because they didn't believe a lot of the things that had apparently happened to their children."

The ongoing struggle to uncover the truth had taken over the lives of his family, he said, and the lives of many other families in the support group they were a part of.

While his daughter's post-mortem examination in Thailand suggested her death was the result of mixing prescription medicine and alcohol, Boyne said he had come to suspect foul play.

He also had concerns about the investigation Thai police led into the fatality.

Boyne Annesley said because the families often got their children's bodies back to their country some time after the original autopsy was completed, it was often too late for another scan to check.

"It's really heartbreaking… and they're on a mission now."

Shortly after her death in 2015, Christina Annesley's brother Aaron told the Herald she was a bubbly, intelligent girl.

Aaron Annesley was based in Glenore, Otago.

"She was one of those people everybody liked, one of those people that never speaks bad about people," he said.

They had kept in touch, despite spending most of their lives in different countries.

The Ministry of Foreign Affairs and Trade had said it was aware of the death of a dual United Kingdom and New Zealand citizen in Koh Tao.