About 14,000 litres of diesel leached into the ground at the prison last month after the hatch of a fuel tank was left open as part of a programme to replace two of the three fuel tanks on site.
An investigation is underway to determine how that happened.
Corrections custodial services commissioner Leigh Marsh said it had expanded the number of sampling bores to 11 and soil testing around one bore where the spill occurred had shown diesel at a depth of three to six metres.
Public health advice remained unchanged, with nearby properties told not to use their water for drinking, cooking or feeding stock or animals.
"Although water testing has so far not detected any hydrocarbon contamination in any bores following the initial detection, the National Public Health Service still needs more information to give confidence that the contamination is confined within the prison boundary.
"We continue to work with them and technical experts towards a return to normal for the small number of affected neighbours reliant on private bore water."
Marsh said the affected residents were being kept updated and provided with water as they needed it.
