After working in Thailand, he retired and settled in neighbouring Cambodia, according to his former boss.

But in December 2017, he was arrested at a resort in the south of Cambodia, along with his girlfriend.

Local newspapers identified the accused pair as Robert Gary Knowles, then aged 72, and his Cambodian girlfriend Meas Phary, 28. The newspapers printed pictures of the two of them with a stash of drugs.

The police were said to have seized six packages of crystal meth, five tablets of ecstasy and nine boxes of aphrodisiacs. Paraphernalia used to consume drugs was also displayed.

Since then Mr Knowles has vanished from public view. His family told The Canberra Times that neither he nor they wanted publicity.

But his former boss says he fears for Mr Knowles' safety in a Cambodian jail. Conditions are likely to be bleak in the extreme, with severe overcrowding and minimal medical aid for a man in his 70s. He may well die in prison.

Mr Harris concedes Mr Knowles was arrested on drugs charges, but says the amounts allegedly in his possession were relatively small and not those of a major drug dealer.

He says that despite the charge, the Australian government should do more for one of its citizens and former employees.

"It is amazing that the Foreign Minister has done nothing since Gary was incarcerated. No trial, no hearing, nothing. He is 73 years old and the alleged amount of narcotics was trivial - he was obviously not a dealer", Mr Harris says.

He wrote to Julie Bishop last year when she was foreign minister, saying "he deserves some special assistance and support from Australia as a result of his subsequent 'downhill' spiral attributable to the Australian government sending him to SE Asia.

"Please give this man some humane assistance. He needs medical and psychological assistance at the very minimum."

In its response, the department wrote: "Please be reassured that his welfare is receiving the concerted attention of consular officials in Phnom Penh [the Cambodian capital] and Canberra."

Responding to a request from The Canberra Times, the Department of Foreign Affairs and Trade said it was "providing consular assistance to an Australian man detained in Cambodia".

In this kind of situation, an embassy would usually visit a prisoner in jail but not intervene. Local laws and punishments prevail.

While Mr Knowles and his family do not want publicity, Mr Harris feels an obligation to bring the plight of his former colleague to light because "he used to work for me and I feel some sense of responsibility for him".

In Mr Harris's opinion, the drugs charge does not mean his colleague should be "left to rot in a prison where his life expectancy is severely shortened".

"It is especially bad to Gary because he was initially sent to south-east Asia by the Australian Attorney-General's Department," he added.

When Australians are arrested on drug-related offences in south-east Asian countries, friends and families of the accused adopt different strategies.

In the case of Schapelle Corby, for example, maximum publicity was chosen. The convicted drugs mule was frequently in the public eye while she spent nine years in a tough Indonesian jail after getting caught with a large stash of marijuana.

Since her release and return to Australia, she has become something of a celebrity in some quarters, posing for cameras as she runs to raise money for charity.

Mr Harris says that, in contrast, his former colleague has been invisible.