Ian Harold King, a former Queensland first-grade cricketer, was rushed to Canberra Hospital for emergency surgery after another prisoner punched him in the face on Sunday morning.

The Canberra Times understands that King was attacked as he mixed with other prisoners during the distribution of medication. But it is unclear how a child sex offender, usually separated from most of the prison population for their own protection, came to be attacked.

Police say they believe a 27-year-old inmate carried out the assault and they were continuing to investigate. The spokesman said the motive for the alleged assault was yet to be determined, but initial reports indicated it was unprovoked.

It is understood that about three prison guards and a number of detainees saw the incident.

Investigators were questioning witnesses on Monday and watching security footage.

The spokesman said police expected to lay charges over the alleged assault.

King was jailed in June 2012 for 12 years for sexually abusing young boys in the 1980s and 1990s.

The 69-year-old former Queensland fast bowler claimed he abused the boys to make them better athletes and to help prepare them for manhood.

But his victims were left traumatised and unable to live normal lives.

The ACT Supreme Court heard that King had performed sex acts on the boys, had touched them indecently and had made them look at pornographic magazines and videos.

He pleaded guilty to 25 sex charges, including sexual intercourse with a child and maintaining a sexual relationship with a young person. He will be eligible for parole in 2016.