SERIAL killer Paul Denyer is being investigated over up to four prison rapes committed in six weeks.

Denyer, serving a life term with 30-year minimum for murdering three women in Frankston, had recently been interviewed over one alleged rape on April 14 at Port Phillip Prison but police are understood to be seeking to quiz him about sexual assaults on up to three other male prisoners.

A prison source said Denyer, 40, who now calls himself Paula and has adopted a female persona, had allegedly targeted inmates with either intellectual deficiencies or disorders in his cell in successive weeks.

"PPP (Port Philip Prison) is trying to keep it quiet ... the three slow guys should never have been in contact, never put into his unit,'' a prison source told the Herald Sun.

"They were easy targets for him and bad police witnesses.''

The prison governor reportedly visited one alleged victim in hospital to discuss the incident, the source said.

"Corrections Victoria takes any allegations of assault very seriously, and refers all allegations of criminal offences and serious incidents to Victoria Police for investigation," a Corrections department spokesman said.

"Maintaining a safe and secure prison system is our highest priority. Prisoner placements are closely monitored and managed to ensure the security and good order of the prison is maintained."

Denyer, who murdered Elizabeth Stevens, 18, Debbie Fream, 22, and Natalie Russell, 17, in a seven-week killing frenzy in Melbourne's bayside suburbs in 1993, had reportedly offered a massage to some of the prisoners before allegedly assaulting them.

In recent years, Denyer - previously incarcerated at Barwon Prison - has been unsuccessful in bids to wear make-up in jail, have a taxpayer-funded sex change and formally alter his name by deed poll to Paula.