Prime Minister Jacinda Ardern and newly appointed National Party leader Simon Bridges have faced off in Parliament's Question Time today over the government's proposal to reduce the country's prison population by 30 per cent, with the PM calling one of Mr Bridges' questions "completely irrational".

The two sparred over the Bail Amendment Act, before Mr Bridges asked a question which didn't sit too well with the Prime Minister.

"Given she is so worried about the prison population, who of the serious recidivist offenders who commit murder, serious sexual and violent offences, traffic or take class A drugs that are spending longer in prison would she let out first?" Mr Bridges asked.

Ms Ardern shot out of her seat to respond.

"A completely irrational question, I would point out that his last leader and last prime minister pointed out that continually building more prisons is a moral and fiscal failure and that is a sentiment this Government agrees with," she replied.

Not to be deterred Mr Bridges pressed on with his line of questioning.

"Will she ensure her governments policy of reducing the prison population by 30 per cent will not harm public safety?"