An Austrian who fathered seven children with a daughter he locked in a cellar for 24 years pleaded guilty to murder and enslavement, reversing an innocent plea on the charges.

Josef Fritzl, 73, had pleaded guilty to incest with his daughter Elisabeth at Monday's outset of the trial but denied murdering their newborn son and enslaving her in the windowless purpose-built cellar under his home.

"I plead guilty to (all) the charges in the indictment," Fritzl told the court, a day after Elisabeth gave 11 hours of video testimony during a closed-door session.

Judge Andrea Humer asked Fritzl what led him to change his plea and he pointed to Elisabeth's detailed statement.

The trial was expected to be completed tomorrow with sentencing the same day. The retired engineer would face life in prison for the death of the twin son through neglect. The boy died shortly after being born in the cellar in 1996.

Prosecutors said Fritzl repeatedly raped his daughter in front of the children marooned underground, using her as if she was his own property. The captive children had never seen daylight.

Fritzl also changed his plea from "partial" to full guilt on the rape charge.

He entered the courtroom this time with his face uncovered, unlike the previous two days, shielded by 10 police officers and again wearing his mismatched grey suit with a blue shirt. Cameras were not permitted to film this time.

Austrian daily Kurier reported Elisabeth was physically present in the courtroom on Tuesday. Fritzl lawyer Rudolf Mayer declined comment but said there were people in the viewing gallery whom he did not identify.

Mayer testified earlier that his client cared for the daughter and children he incarcerated "like a second family," despite describing his actions as "monstrous."

Fritzl's abuses came to light last April when he took 19-year-old Kerstin, the eldest child born below ground, to hospital when she was seriously ill.