Kai Clark, an Asian studies major at the Australian National University, was arrested for unlawful assembly in Hong Kong on May 28 and taken to Aberdeen police station.

The 21-year-old says throughout his 33-hour detention, he didn't sleep for more than 30 minutes at a time.

"I can't say whether it was their intention but I was certainly sleep-deprived," Mr Clark told AAP on Wednesday.

The student said he got into an argument with police early on after they said it would be "too inconvenient" to allow him access to his lawyer during a custody search.

The Australian citizen and Hong Kong permanent resident was searched and told to get changed into a grey tracksuit before being taken into a "waiting room".

"I was told we would be assigned beds, but actually it was a conference room where I would spend (the) next 20 hours sitting on a plastic chair," Mr Clark wrote on Twitter on Wednesday.

The fourth-year ANU student said he was forced to wait hours before seeing his lawyer, who had been waiting at the police station.

Mr Clark was subsequently interviewed by an officer from the Organised Crime and Triad Bureau before being eventually released unconditionally late on May 29.

He was told he was still under investigation and could be rearrested and charged should sufficient evidence become available to prosecute him.

Antony Dapiran, an Australian writer and lawyer based in Hong Kong, last week said police can lawfully hold an arrested person for 48 hours before charging them or releasing them, either on bail or unconditionally.

Mr Clark's 33-hour detention was both legal and "not particularly unusual" in the context of mass arrests, the author told AAP.

"In light of the ongoing protests ... the police have been using arrest as a means of intimidation and crowd control."

Australian Associated Press