Senior managers of the Nauru detention facility have been unable to answer key questions at a Senate inquiry into abuse allegations, frustrating senators who described their lack of knowledge as "extraordinary".

The Senate Select Committee on the Recent Allegations relating to Conditions and Circumstances at the Regional Processing Centre in Nauru held its first hearing in Canberra on Tuesday.

Disturbing accounts of sexual assault and squalid living conditions at the Australian-funded detention centre have been revealed in submissions to the inquiry, which was launched after the release of the Government-commissioned independent Moss Review.

The inquiry's first witnesses were from Transfield Services, the company that won a 20-month $1.2 billion tender from the Australian Government to manage the facility.

But many questions from senators failed to elicit direct responses, with Transfield managers taking them "on notice" — to be answered later.

Greens senator Sarah Hanson-Young, who has been leading the charge in investigating the allegations, pressed the executives for information relating to the sexual assault of a young boy in 2013.

She wanted to know: "Who in your staff was made aware of that allegation and what did they do with it?"

"You must have that information," Senator Hanson-Young said.

But they did not.

"Very happy to take that question on notice ... and provide you the detailed timeline and reporting chain of that particular incident or allegation," Transfield's executive general manager of logistics and facilities management, Derek Osborn, said.

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The committee chair, Labor's Alex Gallacher, asked whether male guards were in the shower areas when women and children were using them — referring to the allegations that guards were demanding sexual favours in return for water.

"Are male staff present when women and children use ablution facilities?" Senator Gallacher asked.

Mr Osborn said: "No — not as far as I'm aware."

"Our practice ... is that male staff are not permitted to be in the shower area."

He added that the guards were provided by Wilson Security, not by Transfield.

When Senator Gallacher asked for the breakdown of male and female guards at the centre, Transfield director Angela-Margaret Williams could not provide the information.

"We have 275 expatriate staff and 277 local national staff," she said.

"In relation to the split between male and female we'll take that on notice — I don't have that information at hand."

Inability to provide information 'extraordinary'

Senator Gallacher directed the managers that, if they knew the information, "you have to answer the question".

"If you're asked a gender balance of 500 people I find it extraordinary that the three people at the table don't know," he told the witnesses.

"So take that on notice that I think that's extraordinary".

Transfield chief executive Kate Munnings said they had appeared before the inquiry "on relatively short notice".

Transfield's Derek Osborn (L) and Kate Munnings at the Senate inquiry. ( AAP: Mick Tsikas )

"We want to cooperate fully with the committee but we want to make sure we provide accurate information, not guesses," she said.

Save the Children's Lee Gordon told Senator Gallacher Transfield was responsible for some detainees at Nauru not being provided with basic items such as clothes or food.

"You're telling me that we can't get personal care items to people in detention, because of logistical issues?" Senator Gallacher asked.

Mr Gordon replied: "Senator I'm telling you that in the past, there have been logistical issues, which have resulted in shortages."

Labor senator Kim Carr asked how many incidents of self-harm and serious assault reported by Transfield occurred in the time the company had been in charge at the centre.

But Mr Osborn again answered he was "very happy to provide that detailed data on notice".

Senator Hanson-Young became frustrated when questioning Mr Osborn about who received the reports of abuse.

Sarah Hanson-Young: When were you first made aware of those allegations? Derek Osborn: I'd have to take that on notice, Senator. SHY: Was it before the Moss Review or after? DO: Senator, I will have to take that question on notice. SHY: Mr Osborne I'm not prepared to take that on notice, you must know.

And while the Transfield manager confirmed the tents asylum seekers lived in were mouldy, he deferred answers about how extensive the mould was and how long it has been a problem.

"There is some mould in the tents, we've got a detailed process to remove it — happy to provide our process and procedure on notice to you," Mr Osborn said.

Managers also took on notice questions about when a regime of random drug and alcohol tests began, who was the head of intelligence inside Transfield operations on Nauru and how often the power failed.

Senator Carr queried their ignorance about such basic information.

"So it's a substantial contract but you can't tell me how often the power fails?" Senator Carr queried.

The committee was set up in March and is due to report by June 15.

Changes already being made to Nauru centre: Dutton

Immigration Minister Peter Dutton has criticised the inquiry, saying the allegations had already been examined by former integrity commissioner Philip Moss and his review's recommendations had been accepted.

The Moss Review recommended the Nauruan government and the Immigration Department overhaul how abuse claims are handled.

"The department accepted all of the 19 recommendations made by the Moss Review and the department has worked constructively with the government of Nauru and service providers to make significant progress on implementation of these recommendations," he said in a statement.

"Even the service providers acknowledged the infrastructure and service improvements made under the Abbott Government and strong endorsements were given by Transfield and Save the Children of the Government's prompt implementation of the Moss Review's recommendations."

Mr Dutton also accused Labor and the Greens of delaying proceedings to ensure there was not enough time for representatives from his department to appear.

"The department's evidence would clearly be an inconvenient truth for this Labor-Greens witch-hunt," he said in a statement.