Labour MP Duncan Webb was thoroughly checked - and cleared - for drugs this morning as part of a demonstration from a Corrections dog newly trained in detecting synthetic cannabis.

"I am slightly nervous," Webb said as he sat in a chair and the dog, called Ninja, sniffed at his feet and pockets.

The demonstration was given to Parliament's justice select committee, where Corrections' bosses appeared to discuss the department's annual report with MPs.

Ninja's handler, Matt Feterika, said that Ninja had been trained to detect tobacco and psychoactive substances, such as synthetic cannabis, and if he found something, he would sit down.

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"I'm really hoping he doesn't sit on you, sir," he said to Webb.

After Webb was cleared, Ninja checked a Corrections staffer who had synthetic cannabis in his pocket - and Ninja duly sat down.

Synthetic drugs have been linked to about 50 deaths in the past year, and the Government has just launched a health-based approach to tackle the issue.

Corrections acting chief executive Rachel Leota said Ninja was one of six dogs trained to detect psychoactive substances - a first for New Zealand and believed to be a first in Australasia.

Leota said while synthetic drugs was an issue across the country, it was yet to be an issue in prisons.

"We just want to be ahead of that curve."

Leota said that the prison muster had gone from an all-time high of 10,820 in March this year to the current level of 9661 - the lowest since August 2016.

Part of the reason for the drop in the muster was a new position of bail assessors to help inmates - sometimes with numeracy and literacy issues, and many who are not even aware bail is an option - with "access to justice".

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National MP Mark Mitchell asked whether there were checks and balances to ensure the assessor wasn't trying to see that bail was granted where it shouldn't be.

Leota said police were also involved in the process to ensure it was a robust process.

National's corrections spokesman David Bennett challenged Leota on whether it was in the interests of public safety to let out about an extra 1000 people, 20 per cent of whom are likely to reoffend.

Leota said Corrections did not let anyone out, as people sometimes finished their sentences or were granted parole by the Parole Board.

But Corrections did everything it could to keep the public safe, adding that reoffending rates had been relatively static over the last decade.

She said Corrections were concerned about the low completion rate - 66 per cent - in community rehabilitation programmes, as well as prisoner-on-prisoner serious assaults, which had doubled in the past year.

"Absolutely it's a concern. It's our duty to keep everybody safe."

She defended an international travel bill of $345,000 in the past year, saying part of it was to enable staff to observe Corrections systems in other parts of the world.