There were 44 reported cases of sexual assault of children in Australian detention centres in the 18 months to July 2014, Senate estimates heard on Monday.

The hearing in Canberra also heard that while the government has acted swiftly to reduce the number of the children in detention, the number of children being detained on Nauru is expected to increase by about 50 per cent.

Drawings from children in immigration detention.

According to the Department of Immigration, all known instances of sexual assault were reported to local police in Australian centres and to the Australian Federal Police for the Christmas Island detention centre.

The 44 cases of sexual assault of children between January 1 2013 to July 10, 2014 do not include any cases of sexual assault on Nauru, Senate estimates heard on Monday.