An Australian camp counsellor labelled a "child predator" by United States authorities has surprised Texas prosecutors by agreeing to serve 40 years in prison for sexually assaulting five young boys at a summer camp.

When Scott Zirus, 26, from Pinjarra in Western Australia, was led into court in Kerrville, Texas, on Friday he was expected to plead not guilty to the assaults, but in an about face that also surprised his lawyer, Zirus admitted his guilt and accepted a plea deal.

Just days ago Zirus rejected the 40-year plea deal.

Prosecutors had vowed to seek the maximum sentence under Texas law of life in jail if Zirus took the case to trial and forced his young victims, aged five and six-years-old, to testify in court.

"If we were going to go to trial Zirus would never see the free world again," Todd Burdick, an investigator in the Kerr County District Attorney's Office said.

Zirus' American lawyer James Patterson said the threat of a life sentence in the Texas prison system weighed heavily on Zirus.

"He had completely rejected the deal and I guess he just got to thinking about it and what the downside was and he decided to take it," Mr Patterson said.

Zirus is eligible for parole in 20 years.

Mr Burdick and other members of the prosecuting team believe a life sentence slapped on one of Zirus' cell mates persuaded Zirus to take the deal.

Zirus shares a cell in the Kerry County Jail with Alfredo Ramirez, a 27-year-old from Kerrville, who fought allegations of sexually assaulting 13 and 14-year-old girls, but lost the case and on Thursday was sentenced to 119 years' jail.

"I think that changed Mr Zirus' mind," Mr Burdick said of the Ramirez verdict.

"Zirus is in the same cell with him.

"All of the sex offenders have the same cell together."

Zirus was working at a boys camp in Hunt, Texas, as a counsellor last year under an international exchange program when the first young victims came forward with the allegations.

Prosecutors originally charged Zirus with assaulting three boys, but five victims eventually came forward.

Zirus also ran a children's camp in West Australia and was involved in the Scout movement.

After Zirus' computer was examined by US authorities Kerr County Sheriff Rusty Hierholzer labelled Zirus a child predator and said Zirus may have Australian victims.

West Australian authorities began an investigation and interviewed 370 children who attended Australian camps involving Zirus. Four alleged victims came forward.

Mr Patterson said Zirus will apply to serve his Texas jail sentence in Australia, but the Kerr County DA's office says he will not be successful.

Mr Burdick said West Australian authorities will also have to wait until Zirus completes his sentence in Texas before he can be brought before the courts in Australia.

"They'll get a crack at him after he goes through the Texas penitentiary," Mr Burdick said.

The Texas victims and their families were relieved Zirus entered the guilty pleas and would never be allowed to be a free man in the US again.

When Zirus serves his time he will be immediately deported to Australia and banned from entering the US.

Zirus pled guilty in court on Friday to charges of aggravated sexual assault of a child, sexual contact with a child and continuous sexual abuse of a child under 14.

In Australia Zirus is accused of eight counts of indecent dealing with a child under 13, one count of procuring and inciting a child to do an indecent act and one count of indecently recording a child.

- AAP