We're being told to look at the inroads being made with some US prisons to determine how New Zealand should progress with our justice system.

Waikeria prison, near Te Awamutu, is set to be upgraded, with a new 500 bed high security jail, and a 100 bed mental health facility - a number of beds much smaller than National's planned mega-prison.

Social justice advocate Kim Workman told Kate Hawkesby there was never any need for a super sized prison.

He says there are several states in the US which have made huge reductions in prison numbers, benefiting the public.

'There's been no impact on public safety at all and, in fact, in the three states that have reduced prison numbers by the largest number the crime rate has gone down. "

Workman expressed shock though at the government's back-up plan should the prison population continue to grow.

Corrections Minister Kelvin Davis says contingency plans include prisoners sleeping on mattresses on jail cell floors.

Kim Workman says that sort of thinking is not good enough.

"Well I'm appalled at that idea and I'm not sure what side of the fence he's on actually when he says that sort of stuff."

LISTEN ABOVE AS KIM WORKMAN TALKS TO KATE HAWKESBY