National leader Simon Bridges says he will not only reinstate a three strikes regime if he gets into power but he will make sure old strike offences still count.

Responding to the Government's proposed criminal justice reforms, Bridges also said he would build more prisons before he "softened" bail or sentencing laws.

Justice Minister Andrew Little is taking a proposal to repeal the controversial three strikes law to Cabinet just over a week.

An Act Party policy that was adopted by the National-led Government in 2010, three strikes ensures that third-time offenders who commit violent or sexual offences receive the maximum sentence and no parole.

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Bridges, a former Crown prosecutor who has made law and order central to his leadership, said a National-led Government would reverse any law change.

"We would repeal it. And what we would also do is legislate so that strikes under this legislation that they repeal count under our future ones," he told the Herald.

"So criminals with first and second strikes are not let off the hook."

That means the clock will not be reset for serious offenders if the law is reintroduced under a National Government.

It was initially thought that Bridges wanted strike offences which occurred even after the law was repealed to also count once National reinstated three strikes.

But a spokeswoman clarified today that only offences committed up until repeal would be considered strike offences and would therefore not be discounted once three strikes was up and running again.

Bridges admitted he had not received advice on the proposed change, which could potentially breach human rights protections.

"Not all retrospective legislation is bad," he said.

"And in this case it would be a situation where the criminal in question knows they've received a strike and would be on notice about the consequence of that."

Little called three strikes a "gimmick" and said its provisions could already be replicated under existing sentencing laws.

The number of "strike" offences has fallen in the years since the law change, though the Ministry of Justice has previously said this cannot be solely attributed to the three strikes law.

The coalition Government's planned repeal of three strikes is part of a series of "modest" changes which will be followed by more comprehensive reform of the criminal justice system.

Other measures include greater use of home detention for sentences of less than two years, and restoring the Sentencing Council to provide consistency in sentencing across the country.

The reforms are part of a goal to reduce the prison population by 30 per cent within 15 years.

Bridges said a National-led Government would also aim to reduce the prison muster, though he did not have a specific number in mind.

It would not get there by changing bail, sentencing or parole laws, which Bridges said were reasonable and proportionate.

National would instead go ahead with an expansion of Waikeria Prison in the Waikato – which the Government has shelved – and focus on rehabilitating prisoners within jail rather than in the community.

The coalition Government has been advised that it will need to build new prisons every three years to keep up with the growing prison population.

Bridges said increased rehabilitation efforts would hopefully mean that no further jails would be needed on top of Waikeria.

"But ultimately if it was a question of going soft on the law or another prison build, I'd choose the prison build every time."

A National Government would also focus on pre-prison programmes designed to keep young offenders from jail, and Rangatahi and Pasifika courts which follow Maori and Pacific cultural processes.