The intruders, thought have broken into the Verner Street structure late Saturday night or early Sunday, broke a 1.3 metre high stained glass window on the southern side above the high altar and threw a tin of gold-coloured paint over 100-year-old parquetry flooring, which was under restoration.

READ MORE:

Goulburn's Sts Peter and Paul's Cathedral project looks to the skies

Goulburn's Sts Peter and Paul's restoration battles pigeons

Goulburn's tourism sector set to 'boom' with Sts Peter and Paul's Cathedral project

They also smashed two bevelled glass windows in the newly created narthex at the Cathedral's rear and stole up to four metres of copper downpipe installed on the southern aspect's exterior.

The latter allowed water into the Cathedral, damaging flooring and paint on the walls, completed last month.

"It's heartbreaking," Mary Queen of Apostles parish priest, Father Joshy Kurien said.

"We were on the finishing line for the project. We were all excited that the Cathedral was going to be re-opened and then this happened, for no reason. I don't know why people do it."

ALSO READ: War nurse, playwright and farmer: Millicent's life inspires theatrical revival

He and fellow restoration committee member, David Bourke, spent four hours cleaning up the paint on Sunday.

But other aspects are more complex and costly.

Project Strategies project manager, Stuart Cunningham, said only a small section of the stained glass window remained. Moss Vale-based specialists had already been commissioned to clean and restore the Cathedral's stained glass windows but they or another firm would have to be enlisted to repair the latest damage.

"We will be struggling to get it done by the time of the reopening, unless they can shuffle things around a bit," Mr Cunningham said.

He estimated a $30,000 cost just for the glass replacement but structural repairs and re-mounting scaffolding for the work could add another $20,000.

ALSO READ: Plein air artist who switched careers showcases hidden landscapes

Likewise, the bevelled glass panels and surrounding trim would need to be replaced and parquetry flooring seals fixed after the water ingress.

The handmade copper piping, sourced from Sydney, cost about $1000 but Mr Cunningham said the thieves would be lucky to get $10 for it from a scrap metal yard.

"Two days of manpower has gone into fixing all this when we could have been doing other things," Mr Cunningham said.

"It's disappointing and very frustrating. I just don't get it - this is other people's property."

Restoration committee chair, Dr Ursula Stephens, also described the damage as "very disappointing" but said it wouldn't affect the reopening.

ALSO READ: Echidna caught roaming around town

The Cathedral was due to reopen on November 30 following a year-long internal restoration project. The estimated $9 million work has also extensively restored the exterior, with sandstone repointing, structural stabilisation, new guttering and downpipes, slate roof and refurbished fleche, among other aspects

Goulburn officer in charge, Inspector Matt Hinton said police were seeking public assistance as part of the investigation. Information can be provided at the Police Station or via CrimeStoppers on 1800 333 000.

Do you have something to say about this issue? Send a letter to the editor. Click here for the Goulburn Post

Did you know the Goulburn Post is now offering breaking news alerts and a weekly email newsletter? Keep up-to-date with all the local news: sign up below.