Police and Hobsons Bay council are investigating after about 20 trees were this month chopped down in Altona North by men who appeared to be testing a chainsaw.

Up to four men wearing hi-vis shirts and wielding a chainsaw were seen by a number of people at the Toyota Nature Reserve along Kororoit Creek on February 25.

Friends of Lower Kororoit Creek president Geoff Mitchelmore said he received reports of the men cutting down trees both in the morning and afternoon.

“This wilful destruction of the environment and the work done by volunteers over 17 years developing this special area is a disgrace and disheartening to the thousands of people who have contributed to this environmental project along Kororoit Creek,” he said. “It is estimated that 20 mature trees (eight-10 years old) would have a nett worth of approximately $10,000.”

Since its inception in 2001, the Friends group has transformed the banks of the creek by planting more than 50,000 trees and shrubs.

The creek is a winter home for the endangered swift parrot and its rejuvenation has resulted in the return of more than 110 bird species.

Friends volunteer Len Towerzey said it was heartbreaking to see the trees chopped down.

“It’s very disheartening considering the amount of work we put in over the years to plant them,” he said. “Of course it makes you angry.”

Mayor Angela Altair said it was disappointing that ratepayers would ultimately foot the bill for the thoughtless act.

“Replacing vandalised plants and trees costs us around $10,000 every year.”