Expanding electronic monitoring of offenders could be at the expense of their rehabilitation in the community, a justice reformer says.

Parliament is considering a bill which would extend the use of electronic anklets to two new sentences. The devices are already used for people on home detention and other community sentences.

Robson Hanan Trust spokesman Kim Workman told a Parliamentary select committee this morning that it was a "watershed" piece of legislation because it could extend electronic monitoring to 5300 more people.

He said electronic monitoring had become "fashionable" in the United States and other jurisdictions, with mixed results.

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"One of the downsides of that is it starts to replace the sort of rehabilitation measures that have been tried and true."

Greater dependence on electronic monitoring often led to a reduced focus on social support and employment, Mr Workman said.

If passed, the bill would expand the use of electronic monitoring to people who were temporarily released from jail and to those who were on "intensive supervision" conditions in the community. It was backed unanimously at its first stage.

READ MORE: More offenders to be GPS-tracked under new plan

Electronic monitoring has come under heightened scrutiny following two high-profile breaches by high-risk sex offenders.

The inquiry into one of those cases, the management of killer Tony Robertson, will include a broader investigation of electronic monitoring for high-risk offenders.

Corrections chief executive Ray Smith defended the electronic monitoring regime at an event at Rimutaka Prison last night.

At present, around 2000 people in the community wore the devices.

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"If they didn't have that on, they would be in prison," Mr Smith said.

The cost of keeping 2000 more people in prison could be used to build two hospitals or many schools, he said.

"The five-year cost of that is $1 billion. There will be some people who break the rules and get it wrong. And we'll all be worried about that.

"What you have to do is you have to work out whether you can do better than that by having people outside of prison."

Mr Smith said home detention was not always a popular policy, but it was Corrections' most successful programme for reducing reoffending.

People on home detention had a recidivism rate of 19 per cent, compared to the overall rate of 30 per cent.