Most prisoners will have to serve two-thirds of their sentence before becoming eligible for parole under prison reforms to be outlined today.

That is double the one-third threshold now faced by most prisoners who serve sentences of two years or more. The changes mean the Government is set to backtrack on a widely trumpeted law change introduced in 2002.

The plan will be recommended in a Law Commission report to be tabled in Parliament today as part of changes aimed at reforming the criminal justice sector.

There are a number of reasons for the proposed parole change, including a belief that public trust in the criminal justice system, at a time when sensitive reforms are under way, will be enhanced by greater "truth in sentencing" - a concept previously heavily criticised by former Justice Minister Phil Goff who introduced the Parole Act 2002.

But the Government does not intend inmates serve longer sentences as a result of the parole changes, an effect which would only increase the problem behind the reform drive: the unexpected surge in the number of people in prison.

Instead it is expected that a new sentencing council will be used to guide judges to reduce actual sentence lengths to counter the effects of the parole changes.

As reported in yesterday's Herald, the establishment of a council (or a similarly named body) is also believed to have been recommended in the Law Commission report and will also be supported by the Government today.

The Parole Act 2002 caused considerable controversy when it was enacted in tandem with a new sentencing act. It made it harder for the worst offenders to get parole, but was designed to create more flexibility in eligibility for other prisoners.

Before, most prisoners serving more than two years were automatically released after serving two-thirds of their sentence. Under the 2002 changes those prisoners could be released after serving a third of their sentence, but some could be made to serve the entire sentence.

Opposition MPs and some law and order groups railed against the one-third provision. They claimed it was a soft and dishonest option, as it could see offenders released after serving only a small amount of their sentence.

Mr Goff strenuously rejected calls for greater "truth in sentencing" at the time, but in April, Law Commission president Sir Geoffrey Palmer slammed the one-third provision as something bordering on "deception".

Two months earlier the commission was given an "urgent" direction by the Government to prepare a report on sentencing and parole laws.

Sir Geoffrey said then that for transparency reasons there needed to be "a closer relationship between a nominal sentence which is imposed by a judge and the actual time served by a prisoner".

The Parole Board's job in assessing eligibility was to determine whether there was any risk to the community, but it appeared to be operating beyond that brief, he said.

It is understood there has been growing concern that the board's unwillingness to release inmates who don't pose a risk, but who it believes have served too short a sentence, means it is effectively "resentencing" them. The changes also resulted in more cases being heard by the board.

Percentage of sentence served by inmates jailed in 2003:

* Males: On average served 52 per cent; 12.5 per cent served full sentence.

* Females: On average served 42 per cent; 12.5 per cent served full sentence.