Australian soldiers on leave from the army are enlisting for lucrative work as guards at Papua New Guinea's Manus detention camp amid claims of drug abuse and thuggish conduct among its newly recruited security force.

The Defence Force has confirmed it has allowed seven soldiers and an unspecified number of air force personnel to work at Australia's network of offshore detention centres.

But insiders say many more soldiers are working at Manus without their superiors' permission, and that illegal steroid use is widespread among the workforce of new security contractor Wilson.

The allegations are the latest problem for the troubled centre, the scene of a night of violence in February that left one asylum seeker dead and 62 injured.

Several Wilson guards have been sacked for inappropriate conduct since the company took over the Manus contract in the wake of the fatal violence, including one man dismissed after a bizarre incident of sexual harassment of a disabled asylum seeker held in the camp.

Wilson Security has defended its record, saying its workers are thoroughly drug tested before being hired, but are not asked to disclose their military status.

Wilson was hired in February for the Manus contract soon after detention centre operator Transfield was chosen to replace G4S, which had recruited its own security guards.

Transfield is being paid $61 million a month to run the Manus detention centre, which holds 1294 asylum seekers, and the Nauru centre where 1166 people are held.

A Transfield spokesman denied there was a culture of steroid use among its subcontracted security workforce on Manus.

''Employees are required to undertake a full medical, which includes drug screening, prior to employment,'' he said.

The Defence Department said its soldiers and air force personnel were acting within the rules if they told their superiors they were working outside the military, but Defence would not confirm if it was investigating claims of soldiers moonlighting with Wilson Security.

The Australian Greens say they have also been approached with allegations of lawlessness, steroid use and inappropriate conduct by expat Australian guards at Manus.

Fairfax has learnt of a Wilson guard who approached a detainee at the camp while holding a banana and suggesting that the asylum seeker should perform oral sex. Wilson confirmed that the incident occurred and that it promptly sacked the guard, although it disputed the version of events supplied to Fairfax.

A former worker at Manus, who has asked not to be identified, said lucrative short-term contracts with Wilson in the detention network were regarded as ''cash cows'' among soldiers and ex-soldiers on Manus. ''… there are a number there under Wilsons who are on leave and using it as a bit of a cash cow,'' the former guard said. ''They've come back from Afghanistan with a heap of leave owed to them.''

The ex-employee said steroid use among recruits was ''common knowledge'' on Manus. ''They're built like German tanks and you don't get built that way by working in a gym.

''You'd hear them carrying on about how much weight they're losing [when] they're not getting their stuff.

''They're talking about how they're going to get it in and whether the local chemist sells them.''

A Wilson spokeswoman conceded there had been several sackings for inappropriate conduct but would not say how many guards had been fired.

''All new staff receive induction training, including training in the Department of Immigration and Border Protection code of conduct,'' she said. ''Training is reinforced by ongoing supervision by area managers and management.

''The number of dismissals of staff is insignificant in relation to the size of the overall workforce.

''Employees are required to undertake a full medical, which includes drug screening, prior to employment.

''Wilson Security applicants are required to disclose any potential conflict of interest prior to employment, all references are checked and approved.

''Military status is not monitored by Wilson Security.''

Greens senator Sarah Hanson-Young confirmed on Friday that she was aware of many of the allegations.

A spokesperson for Immigration Minister Scott Morrison said the government took all allegations against service provider staff seriously.

''Service providers are required to ensure that all of their personnel are of good character and are subject to internal disciplinary processes,'' the spokesperson said.

''All staff employed at the OPC (offshore processing centre) are required to adhere to a code of conduct, however, discipline of service provider employees is an internal matter.''

With Sarah Whyte