Australia’s immigration department regularly interfered with medical assessments on Nauru and asked medical staff to change a report, a Senate inquiry has heard.

Former International Health and Medical Services (IHMS) mental health director Peter Young told a hearing on Tuesday into serious allegations and conditions at the Nauru detention centre that it was a regular occurrence for the immigration department to interfere with diagnoses.

Young said that department officers would “regularly” request changes to medical assessments for asylum seekers at the detention centre, and that there was a general “unwillingness to accept that people had genuine or real mental health problems” arising from their time in detention.

“Officers of the department would frequently in their conversation say that people being taken to Australia for treatment would undermine the policy of offshore detention,” he said.

“When making recommendations to people’s mental health we were told it was unacceptable to put in these reports that detention caused harm.”

Young said he had refused to change reports when requested, and said it was “well established these sort of conditions caused harm to mental health ... and it was right and proper to include that in the report”.

He drew on specific examples, including one instance where the department deterred the reporting of a “botched surgery” at the Nauru hospital to Australian and Fijian medical authorities.

“One of the most striking ones was a case of a botched surgery that occurred in Nauru. There was a patient who had a procedure done at the local hospital. There was a misdiagnosis in that case. There were very serious post-surgical complications,” he said.

“The person had very serious complications and the departmental position on that was we should not be reporting that surgical report that shows the misdiagnosis.”

“They said we shouldn’t be reporting that to the Australian health regulatory authority. They were very reluctant to do anything about it. They said we shouldn’t be referring it to the Fijian health authority either.”

He said the “only reluctantly and after considerable pressure” did the department permit the allegations to be referred to the Nauruan health authorities.

Speaking generally about the problems with the immigration detention environment, Young said: “Where there is a power differential between those who are being supervised and those who are supervising you create situation where abuses can tend to occur.

“There are conditions in the offshore processing centres that cause harm to peoples mental health, and do so greater than Australian detention centres.”

The inquiry also heard from former magistrate Peter Law, who was deported by the government in January 2013 after he ordered an injunction relating to the deportation of two Nauruan residents.

In testimony to the committee, Law said he received a text message from the Nauruan justice minister, David Adeang, about the case just a week earlier.

The allegation would amount to an extraordinary interference with judicial processes.

Law said: “After I did so I received a text message signed DA, I presume that was the minister for justice, David Adeang.”

“That text message says ‘don’t you realise I’m authorised by cabinet to make this decision.’”

He was also concerned about the ability of the Nauru police force to investigate serious assaults and abuses, since the departure in early 2013 of the Australian federal police officerwho was the Nauru commissioner of police.

“They [the AFP] were able to offer their expertise and assurances of independence in the investigations.”

“After the departure … I had sent off matters for investigation as my role as coroner. In fact I had no reply from the newly made director of police, despite follow ups.”

“It left me without any confidence that the NPF would act either independently or with the capability of capacity they really needed.”

His comments follow remarks from a former senior adviser to successive immigration ministers, Caz Coleman, who said asylum seekers who have been sexually abused on Nauru may never face justice.

On Monday the ABC also reported that the Nauruan president, Baron Waqa, and Adeang at the centre of bribery claims.

Law reiterated concerns about a serious breakdown in the rule of law on Nauru, following a series of deportations by the government, the suspension of opposition MPs and the introduction of a new offence that could stifle dissenting voices.