HONG KONG, March 10 (AFP) - A Hong Kong air-conditioning repairman has been jailed for life for killing a Thai woman and hiding her body in the city's main tax office, a court official said Wednesday.

So Kam-tong, 27, was found guilty of stabbing and suffocating Charitar Kamolnoranath, 40, in 2005 and then hiding the Thai tourist's body on top of a ventilation duct at the headquarters of the city's tax collection agency.

A seven-person jury unanimously convicted So -- who is serving a separate life sentence for the 2008 murder of a female airline employee -- on Tuesday, the official said.

The case remained open for years until a friend of So's, who admitted he helped hide the body, agreed to testify under immunity from prosecution, media reports said.

Charitar, who was in Hong Kong to visit her Australian boyfriend, was the daughter of a prominent Thai gemstone dealer, reports said.

The court heard that So rode the lift in the tax office looking for a target. "The deceased in the present case was purely a random victim," Judge Clare-Marie Beeson was quoted as saying.

So's defense argued that he had accidentally stabbed Charitar during a botched robbery.