A diesel leak that closed Queenstown's civic building for a fortnight last year cost $35,500 to clean up.
Up to 100 litres of diesel flowed into the Queenstown Lakes District Council's main offices in Gorge Rd from a rooftop tank on November 5.
A report for tomorrow's audit, finance and risk committee meeting says the leak came from the main unit of the library's in-floor heating system.
Property adviser Daniel Cruickshank said a faulty probe on a small header fuel tank caused it to keep filling, despite it being full.
The fuel soaked carpets, walls and fittings, forcing about 50 staff to work from home or other council offices. The public library on the ground floor was unaffected and reopened two days later.
Mr Cruickshank said the heating unit, which was installed in the 1990s, had no measures for preventing fuel from entering the building in the event of a leak.
''From a visual point of view, it was not obvious a leak from the unit would cascade into the building, but it's clear the unit would not be installed in this way under current best practice.''
The unit had no major issues in the past, and servicing contractors were ''unlikely to catch a flaw in the system without a separate review of all aspects of the equipment''.
The council received an insurance payout of $17,600 for the costs of the clean-up and repairs.
The opportunity was taken while the building was closed to further investigate a double-storey limestone fireplace that a recent structural engineering report had identified as having a potential structural weakness, he said.
The fireplace was removed after it was found it could ''topple or break up in a significant earthquake''.
