An Australian man whose girlfriend was killed after the couple's jet skis collided in Thailand is "so broken" by her death and says he will "never be able to mend".

Emily Jayne Collie - a 20-year-old from Kyabram in central Victoria - died from head, neck and shoulder injuries after her jet ski collided with one hired by her boyfriend, Tommy Keating, off the coast of Phuket on Sunday.

Keating, 22, said he was blinded by the sun's reflection on the water just before running into Collie.

Thai police could lay charges against the heartbroken boyfriend, who said Collie was "the most beautiful girl I've even met. Inside and out".

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"I love you so much Emily and I wish I could just bring you back into my arms," Keating posted on Facebook. "I'm so broken and I know I'll never never be able to mend. "I'll always be your boy and you'll always be my girl!"

The university student was pronounced dead at hospital after lifeguards dragged her from the water and performed CPR for 15 minutes.

A Thai legal source told AAP that "the bigger issue is clear, [it] is about the cause of the accident".

Police could charge Keating with reckless driving causing death, Fairfax Media reported today.

Keating's sister Bree Lyon told Fairfax his family was "absolutely devastated" about the death of "our darling girl" with a "beautiful smile".

Friends of Collie, who went Charles Sturt University, used social media to post tributes.

"Emily Jayne Collie you will forever be loved, hope there's a nice big trampoline up there for you xxxx," one woman wrote.

A Department of Foreign Affairs and Trade travel advisory warns there have been many serious accidents involving jet skis and motorcycles in Thailand. DFAT is providing consular assistance to Collie's family.

- AAP