Christchurch Men's Prison. Photo: Luke McPake / The Wireless
Corrections said water testing near Christchurch Men's Prison after a fuel leak continues to return clear results.
About 14,000 litres of diesel leached into the ground at the prison on 24 May after the hatch of a fuel tank was left open, potentially affecting the water supplies of neighbouring properties.
Corrections custodial services commissioner Leigh Marsh said groundwater tests from inside and outside the prison are coming back clear.
There was one positive result for dissolved hydrocarbons in a bore sample from outside the prison on 28 May.
National Public Health Service advised eight properties neighbouring the prison to stop using water from their private bores last Friday.
Marsh said that advice remains unchanged.
The overflow occurred on 24 May but was not found until the following day, when diesel was found in cable pits and underground infrastructure, but there was no evidence at that point it had seeped into the ground.
However, on 30 May, it was found diesel might have soaked into the ground through a crack in the floor of a switch room.
Marsh said the incident was being investigated and the exact cause was not yet known, but it appeared to be related to the installation and change over to a new fuel tank.
Corrections were working with technical experts, while affected residents were being kept updated and provided with water if needed.
He said the number of sampling bores has been increased, with new points added to support the National Public Health Service's decision on when normal water use can resume.
Marsh said it was taking a cautious approach, with Christchurch Men's Prison operating on the water supply from Christchurch Women's Prison to reduce risk.
The water regulator, Taumata Arowai said the initial water testing results provided by Corrections did not exceed New Zealand's Drinking Water Standards but it would continue to monitor the situation.
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