An Australian man has pleaded guilty to reckless driving over the death his girlfriend in jet ski accident on Thai resort island of Phuket.

Thomas Keating, 22, accompanied by his family and relatives of Emily Collier, admitted guilt to Thai police late on Thursday to the death of Ms Collier as a result of the accident.

Keating and Ms Collier were riding jet skis near Kata Beach late in the afternoon Sunday when they collided at high speed.

Keating said glare on the water from setting sun had impaired his vision.

Thai police told reporters the couple were trying to splash each other when the accident occurred.

Ms Collier suffered severe neck and should injuries. The 20-year-old was later pronounced dead at the local hospital.

Police said it had been the first time Keating had ridden a jet ski.

During the seven hours while investigators questioned Keating, he accepted a plea of guilty, Karon police chief Sanya Thongsawad told Phuket News on Friday

Col Sanya said Keating told investigators a large wave surged while they were zig-zagging, causing the two jet skis to collide.

"Excepting the charge is a good thing because the sentence will be reduced," he said

Ms Collier's family said they did not apportion blame to Keating for the tragedy.

"We are all heartbroken beyond words and at this extremely sad time we wish to be together with our family, with Tom and with other loved ones, to grieve and celebrate Emily's life," they said in a statement.

Ms Collier was a student at Charles Sturt University and a resident of Kyabram, north of Melbourne.

Her death has been marked by an outpouring of emotions on social media by friends and family.

Keating posted a message saying he felt "broken" and would "never be able to mend."

"I love you so much Emily, and I wish I could just bring you back into my arms," he wrote.

The Australian Department of Foreign and Trade said it was providing consular assistance to Keating and his family.

DFAT, through its Smarttraveller website, posts regular warnings to Australian travellers over the risks of riding jet-skis in Thailand, including issues related to insurance cover.

Following the accident, members of the Royal Thai Navy, Marine Police and Kata Police found six jet-ski operators working from Kata Beach were renting craft without the required insurance.

Thai police have expressed sympathies for the Australian families during their plight.

Col Sanya told local media the police had no choice but to press charges of reckless driving against Mr Keating.

"By law in Thailand, when an incident results in the death of a person, police are required to formally press a charge," Col Sanya said.

The Thai public prosecutor is now preparing a case expected to be presented to court within the next fortnight.

Thai media reports in Phuket say Mr Keating may face a suspended jail sentence after the guilty plea and could return to Australia within four weeks.

But under Thai law the charge could result in a jail term of up to 10 years.

© AAP 2017