The state of NSW had applied to the Supreme Court for a two-year continuing detention order for Wayne Lindsay Wilmot.

The 46-year-old was due to be released on June 26 after serving a 20-year jail term for other kidnapping and sexual assault offences against multiple victims.

On Tuesday, however, Justice Julia Longergan granted the state's application for the CDO, which will expire on September 23, 2021.

It has been a brief "moment of relief" for Ms Balding's brother David Balding, who still lives in Wagga.

However, with an expiration date on the CDO, Mr Balding said he and his family are still dealing with the burden of knowing Wilmot could be released in the future.

"It comes around and we just have to deal with it," he said. "All four of them should be dead as far as I'm concerned."

Wilmot's extensive criminal history, which started with a sex attack when he was 13 years old, includes serving eight years for the abduction and rape of Ms Balding.

The 20-year-old bank teller was grabbed from a Sydney train station by five homeless youths before she was repeatedly raped.

Wilmot, who was a teenager at the time, remained in the car when Ms Balding was then bound, gagged and thrown over a fence before she was held underwater in a dam until she drowned.

Wilmot was given a lesser sentence for the kidnapping and assault of Ms Balding, then released in 1998 for a short time before being jailed again.

He has committed sexual offences against women and men, involving at least eight victims including a woman he beat, bound and raped at another Sydney train station in 1998.

Victim's advocate Howard Brown said extending Wilmot's jail time by two years is an "excellent result", but knowing he might be released in two years creates "a level of anxiety" for not only Ms Balding's family, but his other victims.

"Wilmot clearly has a number of deficits and that requires treating, so in his interest it's best to detain and work on him to reduce or diminish offending," he said.

"Clearly it was preferable that he was given a life sentence, but it's a burden we bear as victims."