The Department of Primary Industries has confirmed 23 myrtle rust infestations in Victoria since December.

The fungus poses a threat to Victoria's nursery, forestry and beekeeping industries and also public parks, gardens and native forests. It attacks species in the Myrtaceae plant family including gum trees and bottlebrushes.

Department myrtle rust incident controller Gordon Berg said affected sites were mostly in Melbourne around the Mornington Peninsula but there was also one at Lorne.

"At the moment we are in a containment program. We have issued infested land notices to affected sites," he said.

"People have the option of treating or destroying the plants. Most of the premises have been cleared.

"Hopefully that will delay it getting into native park lands, which is a major concern."

Mr Berg said it was critical that anyone who saw suspected myrtle rust called the department immediately.

"We ask people not to take samples and inadvertently spread the fungus. We suggest people take a photo and send it to us," he said.

Stawell botanist and environmental consultant Neil Marriott said myrtle rust was a very serious issue.

"It is called a rust because that's what it looks like on the leaves and foliage," he said.

"It grows inside the plant and germinates into the new growth and releases millions of spores.

"Myrtle rust prefers coastal, humid conditions but because of the amount of summer rain we've had in the past two years it is spreading at an alarming rate.

"If it got into the Grampians it could devastate the gum trees."

Myrtle rust spores can spread on contaminated plant material, equipment, vehicles, clothing and shoes and through wind and water.

Mr Marriott said the disease had the potential to wipe out Australia's koala population because it attacked gum leaves, a food source for koalas.

"The important sacrifice we need to make is to not buy or spread any young gum tree plants because they could be infested," he said.

"You don't know you have it on a plant until it germinates."

Mr Berg said people could call the department on 1800 084 881 to report sightings or email photos to plant.protection@dpi.vic.gov.au.