The group – seven of whom are former Australian Defence personnel – are part of the emergency response team on Nauru and are employed by Wilson Security.

A spokesman for Transfield Services, which engages Wilson staff as subcontractors, said the Facebook posts by the men – which included a group picture with Pauline Hanson at the Reclaim rally in Brisbane on April 4 – were "totally unacceptable" and would be thoroughly investigated.

A spokesman for the Immigration Department said service provider staff were expected to "act appropriately and with integrity in all their dealings with the people in their care".

"Inappropriate use of social media channels will not be tolerated, particularly if it involves offensive material," he said.

In pictures posted on Facebook, two of the men, Dan Connors and Beau James, wear T-shirts that proclaim them as "Australian infidel" and "infidel", respectively.

Guardian Australia reported another guard, Harley Levanic, had posted a picture of himself on Facebook displaying his new neck tattoo proclaiming himself an infidel.

James commented on that picture, saying: "Welcome to the gang … well done old son. Looks awesome."

Another guard, Dan Connors, linked to a story about a woman beaten to death in Afghanistan with the comment: "Theres [sic] a lot of reasons why I think islam is a disgusting and evil religion … Absolutely disgusting and needs to stay out of our country."

When the emergency response team was recruited, part of the job requirement was "cultural sensitivity".

Transfield Services spokesman David Jamieson told Fairfax Media that the investigation was looking at a range of company policies that are likely to have been contravened.

It has been reported that the alleged offence was breaking Wilson's social media policy and the picture with Ms Hanson was the central issue.

But Mr Jamieson said the investigation was "much broader".

"It's totally unacceptable. It's not representative of the broader workforce. We are very, very concerned about this," he said.

Mr Jamieson said appointment to the emergency security team required "acceptance of diversity and several cultures".

He would not comment on how many members comprise the Nauru security team.

The scandal around guards on Nauru comes as the Abbott government is pushing ahead with its Migration Amendment (Maintaining the Good Order Of  Detention Facilities) Bill 2015.

The bill, if passed, will give security staff greater powers to deal with detainees. The explanatory memorandum of the bill says that detention staff are dealing with "increasing numbers of high-risk detainees, including persons who: have had their visa cancelled as a result of failing the 'character test' often due to convictions for drug and other serious criminal offences; are a high security risk, such as members of outlaw motorcycle gangs; are subject to adverse security assessments; and have become unlawful non-citizens as a result of breaching certain visa conditions".

"The Bill amends the Migration Act 1958 (the Migration Act) to allow an authorised officer to use such reasonable force against any person or thing, as the authorised officer reasonably believes is necessary, to: protect the life, health or safety of any person (including the authorised officer) in an immigration detention facility; or maintain the good order, peace or security of an immigration detention facility," it says.

The government said a number of disturbances in detention warranted a greater scope for force.

Those disturbances include the riot inside the Manus Island detention centre last year in which Iranian asylum seeker Reza Berati was beaten to death by guards. Two Papua New Guinea men are due to stand trial for his murder.

Immigration Minister Peter Dutton has been sought for comment.

Greens immigration spokeswoman Sarah Hanson-Young said the government should review training for security staff at detention centres.

"You've got to wonder what screening and training these staff have undergone. The government must take responsibility for the actions of these guards," she said.

"It's time there was a review of all training and screening requirements for staff employed in immigration detention centres."

"It is clear that these guards have no regard for the people they are supposed to be looking after."

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