Patients and staff at a Christchurch hospital are being provided with bottled water after the site's drinking supply was found to be contaminated.

Canterbury District Health Board confirmed tonight that water at Princess Margaret Hospital, which houses mental health patients including mothers and babies, was unsafe to drink.

In a statement, it said a contaminant was identified during routine water testing of the hospital's private supply.

"As a precaution, we have advised staff, consumers and visitors not to drink the water from taps, zips or plumbed-in water coolers until further notice," it said.

It was providing bottled drinking water and are treating the site's water supply with chlorine. It said the measures would stay in place until the issue was resolved.

Princess Margaret was due to be decommissioned last year. It has been described as "derelict" and "appallingly depressing", "substandard" and "not appropriate".

The hospital was opened in 1959 and is situated at the foot of the Port Hills, in the south of Christchurch on Cashmere Road.