More parole-eligible prisoners are being kept behind bars and those who are released are committing less crime.

An analysis of Parole Board decisions shows that despite an increase in the number of parolees, the number of people committing crimes while on parole has more than halved - dropping from about 100 a month in 2002 to about 45 a month.

"That's hugely gratifying, but we know that we live on the edge of risk, and that many other factors can influence the outcome, even when the best decisions are made. We take these figures as an opportunity to look for further improvements," Parole Board chairman Sir David Carruthers said yesterday.

He said the results of the research, which focused on offenders who were released between July 1999 and June this year after serving prison sentences of more than two years, surprised even the Parole Board.

"We've been looking for proof that what we're doing is achieving the best possible outcome, but these statistics are even better than we had anticipated. They are in line with the best international figures - and show that with the right risk assessments, parole works."

The analysis also revealed a big increase in the number of parole-eligible prisoners being detained for more than 70 per cent of their imposed sentence. The number of those prisoners has increased from about 10 per cent in 2003 and 2004 to nearly 50 per cent now.

The Department of Corrections' manager of strategic analysis and research, Peter Johnston, said the change from mandatory release at two-thirds of sentence had led to considerably less offending occurring among offenders released from prison early.

Sensible Sentencing Trust spokesman Peter Jenkins said the Parole Board had become more "risk-averse" since the Graeme Burton "disaster".

Burton killed Wellington man Karl Kuchenbecker in January 2007 while he was on parole for another murder.

"They're a lot more careful; they don't let as many people out," he said.