Prison breaks usually involve inmates trying to escape, but an unusual variation last night saw thieves break into New Plymouth Prison to make off with a plasma screen television.

Police were alerted to the unconventional heist after a fire at the historic central city jail set off sprinklers about 11.50pm.

Sergeant Thomas McIntyre said firefighters arrived to find curtains inside an administration building had been deliberately set alight.

Police were called after firefighters noticed a window had been forced open.

Mr McIntyre said the only thing stolen was a 50 inch plasma TV.

The thieves probably left the area on foot, taking the nearby Huatoki walkway.

"If any members of the public saw anyone carrying a big TV at that time of night, or heard or saw anything in the area of the prison, let police know," Mr McIntyre said.

New Plymouth Prison is the oldest operating jail in the country, having been in continuous use since the 1860s when it was converted from an army hospital built for the Taranaki land wars.

The level of security at the prison varies from building to building, according to the Corrections Department website.

The old jailhouse is surrounded by a large stone wall topped with razor wire, while a newer unit is surrounded by a fence topped with barbed wire.

Security measures include searches, dog teams, electronic security devices, cameras and closed circuit television.

The prison accommodates 112 minimum- to high-security prisoners.

A Corrections Department spokeswoman was not able to immediately comment.