Prison staff will from today have greater powers to search prisoners and their mail for contraband.

The Corrections Amendment Act, which comes into effect today, also increases penalties for prisoners using contraband as well as for those found to be supplying illicit items, Corrections Minister Judith Collins said.

The Act makes it an imprisonable offence for a prisoner to possess or use an electronic communication device such as a cellphone, and for anyone to possess an unauthorised item without reasonable excuse while visiting or working in a prison.

"Smuggling drugs, cellphones and other contraband into prisons just got a lot harder," Ms Collins said.

Incoming and outgoing mail was to come under greater scrutiny.

She said the legislation would enhance the security of New Zealand prisons and make it more difficult for prisoners to access drugs and use cellphones to continue their criminal activities and intimidate people in the community.

Improved access to drug and alcohol rehabilitation programmes were part of the Government's efforts to reduce prisoner reoffending, and Ms Collins said the Act would make such programmes more effective by reducing prisoners' access to illegal drugs.

A Corrections Department spokesman said cellphone signals were in the process of being blocked at prison sites so cellphones would not function.

Under the Corrections Act prisoners were entitled to a make at least one five-minute phone call on a pay phone per week.

Every prison unit or wing had a prisoner pay phone and inmates could buy calling cards and use the pay phones during "unlock hours".

Prisoners were restricted to phoning 10 pre-approved numbers, with the department checking the owners of those phones that they "are happy to take the calls", the spokesman told NZPA.