Parks Victoria Rangers were shocked to discover that the most significant casting pattern of the centre hub of the water wheel had been stolen.

"The theft is believed to have occurred sometime between Sunday afternoon March 23 and Tuesday 25, and was reported to the local police," Karen Doyle Ranger in Charge, Southern Goldfields, said.

The existence of such early casting patterns is extremely rare and it's thought that those at Anderson's Mill may be the earliest surviving set of casting patterns in Australia.

The mill is listed on the Victorian Heritage Register and the casting patterns are included in the designation as significant heritage objects.

Removal the casting pattern from the site is an offence under the Victorian Heritage Act 1995. The mill is also classified by the National Trust.

"Many will be deeply saddened by the loss of this important historic artefact, which had been cared for and retained on site by the Anderson family and more recently by Parks Victoria for over 150 years," Ms Doyle said.

The five-storey bluestone mill, built in 1862, is located on Birch Creek in Smeaton about 30km north of Ballarat.

The mill was powered by an impressive 25 ton iron water wheel, which was manufactured locally by the Hunt and Opie Victoria Foundry in Ballarat in 1861.

The mill was purchased by the state government in 1987 as a bicentennial gift to the people of Victoria and is managed by Parks Victoria.

Parks Victoria and the local community celebrated its 150th anniversary last year . Anyone with information should call Crime Stoppers on 1800 333 000.