Bangkok: While accused Australian spy James Ricketson languishes unwell in one of Cambodia's harshest jails the Turnbull government insists it cannot intervene in his case.

The Department of Foreign Affairs says its consular staff cannot provide legal advice or intervene in legal cases to get Australians out of prison overseas "just as foreign embassy officials cannot do so in Australia."

The policy which the department says applies to all Australians who get into trouble overseas means that Canberra cannot tell Cambodian authorities that 68 year-old Ricketson is not an Australian spy, if that is the case.

The department reiterated its policy to Fairfax Media as some of the Sydney filmmaker's family members and supporters circulate an on-line petition demanding that Australia intervene to avoid what they say is a false prosecution.