Police on Tuesday said they had arrested a man on suspicion of murder after a 25-year-old woman's body was found in a shallow beach at the bottom of Waterfall Bay in Aberdeen.

The male suspect's wife was also arrested for allegedly aiding an offender.

The force said it had received a report at around 7:30am on Monday. Emergency personnel rushed to the scene, but the woman was pronounced dead on site.

Speaking at a press briefing, superintendent Sin Kwok-ming said the victim was a domestic helper from Southeast Asia.

She had injuries on the back of the head and forehead, but an autopsy will determine her exact cause of death.

Initial evidence suggested the woman arrived at the scene with the male suspect in a taxi late on Sunday night.

The man then left alone about 30 to 40 minutes later, before heading to the mainland with his wife.

Police eventually locked down their identities and arrested them when they returned to the city on Tuesday afternoon.

Sin also explained why the case wasn't classified as a murder at the beginning.

"The arrested man's actions raise a lot of questions, like why the police weren't called right away and why he left Hong Kong immediately afterwards," he said.

"Some evidence has also gone missing for no good reason. All these factors lead us to believe that the woman's death wasn't due to natural reasons."

Sin added the victim had known the man for some time, but was not employed by the suspect, a foreign national holding a Hong Kong identity card.