An immigration detainee who alleged a Serco guard sexually assaulted him has had “incentive points” docked after Guardian Australia published details of the complaint.

In response to the allegations – which have apparently not been referred to law enforcement – Labor and the Greens have warned the government to ensure detention centre employees are not “abusing their power”.

The detainee had accused a member of the Serco emergency response team at the Yongah Hill facility in Western Australia, of coming up behind him at the medical dispensary on Sunday and holding his genitals against him and grabbing his buttocks. He also alleged intimidatory behaviour following the incident, and filed two separate complaints.

Guardian Australia is aware that he has been told his complaint was referred to the Australian federal police but neither the AFP nor the Western Australian police appear to have received it.

On Wednesday afternoon the detainee was given a notice on Serco letterhead, signed by a welfare and engagement manager. He said it was delivered by a guard who also made a comment about his media attention.

“This letter is to inform you that due to the assault incident in which you were involved on the 16.09.18, you are not eligible to obtain your 10 IAP incentive points for this week,” it said.

Detainees receive a certain amount of points each week to spend on cigarettes, food, and phone cards.

“Detainees are not punished for reporting claims of assault and this detainee was not denied IAP points for reporting the incident you have described,” a spokesman for ABF said.

“As we have stated previously, the ABF takes all reports of assault seriously and refers them to appropriate law enforcement agencies for investigation.”

Guardian Australia understands there is CCTV footage of the incident and that management have questioned the seriousness of it.

The detainee, as well as a number of others, have frequently complained about the behaviour of some guards, including allegations of racial abuse and goading of detainees who are on regular watch after suicide attempts.

“Peter Dutton must ensure the health, security and welfare of all individuals in Australia’s onshore immigration detention and transit centres,” said Shayne Neumann, the shadow minister for immigration.

“The minister must take all appropriate steps to guarantee those employed to enforce good order and ensure safety in immigration detention centres cannot and must not be abusing their power.”

Greens senator and spokesman on immigration, Nick McKim, said the assault allegations were “extremely concerning, and must be urgently investigated by the AFP”.

“If this allegation has not been referred to police, Dutton needs to step in and ensure that it happens,” he told Guardian Australia.

“The guard at the centre of these allegations should be stood down until these allegations are dealt with. Dutton cannot hide behind private contractors – people in detention are in his care and remain his responsibility. He needs to step up.”

McKim, raised the matter in question time, asking why the allegation had not been referred to the police, and what steps the government had taken to ensure it was.

“You would know that obviously we do not comment on individual cases, but what I can say is the ABF takes all reports of assault seriously and refers them to appropriate law enforcement agencies for investigation,” Coalition senator Michaelia Cash said, representing the immigration minister.

“I’m advised that the department and its service providers have policies and procedures to appropriately respond to incidents in immigration detention, including alerting relevant law enforcement bodies.

“The department does not tolerate abuse or assaults by staff, contractors, or detainees, within immigration detention facilities. The department acts swiftly and decisively to ensure the safety and security of all detainees and staff.”

McKim asked for confirmation that detainees in onshore detention could not make complaints to ABF or Home Affairs officers, only to Serco staff, and that Serco staff did not wear name or other identification badges.

“How can you be sure that this matter has been referred to the police, and how can the minister possibly fulfil his duty of care to detainees?”

Cash repeated her previous answer, but took questions on notice.

Guardian Australia has sent questions to Serco; they directed questions to ABF.