The Defence Force has confirmed it has given approval for seven serving soldiers and an unspecified number of air force personnel to work as guards at Australia's network of offshore detention centres.

But insiders say there are many more soldiers working at Manus without their superiors' permission, and that illegal steroid use is widespread among the workforce of the 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 security contract after the fatal violence, including one man dismissed after an incident of sexual harassment to a disabled asylum seeker at the camp.

The security company defended its record saying its workers were thoroughly drug tested before they were hired but were not asked to disclose their military status.

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

Transfield is now being paid $61 million a month to run the Australian Immigration detention centres on Manus which holds 1294 asylum seekers and Nauru where 1166 people are held.

A spokesman for Transfield said there was ''absolutely not'' 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 airmen were 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 Australian guards at Manus.

Fairfax has learnt of a Wilson guard who approached a detainee at the camp while holding a banana and suggesting the asylum seeker, who is short, should perform oral sex on the security worker.

Wilson confirmed 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 that 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 the new 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,'' the source said.

A Wilson spokeswoman conceded there had been several sackings for inappropriate conduct but would not confirm 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.

''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

''There is a sense of lawlessness … that hangs over the detention camp,'' Senator Hanson-Young said.

Immigration Minister Scott Morrison said he took all allegations seriously and that service providers were required to ensure that "all of their personnel are of good character and subject to internal disciplinary practices".