A former guard at an Australian youth detention centre has admitted filming himself asking teenage inmates for oral sex.

Conan Zamolo's testimony came during a royal commission inquiry into youth detention in the Northern Territory.

The commission was launched last year after photos showing inmates being strip-searched and placed in a spit hood drew widespread condemnation.

Mr Zamolo said his requests for oral sex were "a joke".

"I was actually just going to say goodnight to them because I was leaving shift," Mr Zamolo said on Monday.

"I knew they'd take it as a joke, or I just assumed they would take it as a joke."

Mr Zamolo also admitted to urging another inmate to eat faeces, and to filming a boy urinating.

"At the time I didn't think it was inappropriate, but now I see how inappropriate it is," he said.

He claimed he had a good relationship with the detainees.

The incidents happened at the Don Dale Youth Detention Centre, where Mr Zamolo worked from 2012 to 2014.

The Royal Commission into the Protection and Detention of Children in the Northern Territory follows widespread condemnation of the treatment of detainees at Don Dale in 2014 and 2015.

In 2015, Amnesty International described the regime at the centre as "institutionalised brutality", with teenagers being held in solitary confinement with no access to light or water for long periods.