One of the country's worst serial rapists is appealing to remain eligible for release next year.

In April, Malcolm Rewa was sentenced at the Auckland High Court to 10 years in prison, after pleading guilty to raping a 16-year-old in Onehunga in June 1988.

He was already serving a preventive detention sentence, with a minimum non-parole period of 22 years, after being convicted of sexual attacks on 25 women.

Rewa, now 73, was also serving a life sentence for murdering Susan Burdett in 1992.

His sentence for the Burdett murder meant he was eligible for release in 2027, but this year's sentencing extended that eligibility out to 2029.

In the Auckland Court of Appeal on Thursday, Rewa appealed the April sentence, asking to keep a parole eligibility date of next year.

Lawyer David Allan said Rewa's sentence did not need an extension, given he was already serving an indefinite sentence of preventive detention.

Allan said the court should consider the totality of the case.

"To extend the parole period out any further than the current eligibility date of 6 February 2027 is manifestly excessive and a disproportionate punishment," he said.

Rewa was already subject to an indeterminate sentence for the same series of offending and becoming eligible for parole did not necessarily mean he would be released on February 6 next year, Allan said.

The most recent sentence came after the now-adult survivor of the rape - who did not know who her attacker was at the time - asked police to run her medical swabs against its DNA database, revealing a match with Rewa.

Allan said it wasn't clear why the 1988 rape had not been merged with Rewa's other rape cases, as was done in 1996.

Crown lawyer Zannah Johnston said the court should look at the 1988 case separately, rather than grouping it with the rest of Rewa's crimes.

"It doesn't follow that a court sentencing in 2026 must only sentence that which would have been imposed back in the 90s or 80s, if this had come up earlier," she said.

"This offending has come to light later and does warrant separate recognition."

The panel of Justice David Johnstone, Justice Grant Powell and Justice Paul Radich reserved their decision.

Another man, Teina Pora, was wrongfully convicted of Susan Burdett's rape and murder in 1994.

He was found guilty again at a retrial in 2000, but eventually had his convictions quashed by the Privy Council in 2014.

A police report from 2006 estimated Rewa may have been involved in up to 26 other sexual attacks.