The world's oldest working steam engine, a priceless object in the collection of the Powerhouse Museum, has stopped running as questions mount over maintenance priorities at Ultimo ahead of the museum's relocation to Parramatta.

The Boulton and Watt Engine, said to have been one of most significant technological artefacts ever to reach Australia, has been inoperable since September 6, along with other steam objects on public display, following the replacement of the old Lucas boiler and redundancies of expert maintenance staff.

Peter Denham, director, curatorial, collections and exhibitions at the Powerhouse Museum in front of the Boulton and Watt steam engine in 2017. Credit:Ben Rushton

Compounding staff concerns is a major water leak on August 24 that affected 25 objects from its renowned collection, two of them permanently, the Public Service Association said.

The water-damaged items included items from the Jenny Kee and Linda Jackson collection, a bridal gown and gifts from the Oceanic Research Foundation.