A 70-year-old London flame tree will have to be cut down from the South Perth foreshore after a mysterious vandal attempted to cut through its trunk in the early hours of Tuesday morning.

City Environment manager Bruce Moorman said the City of South Perth would be conducting a full investigation into the vandalism.

"Someone's come in the middle of the night and put a chainsaw to it," he said.

"We're in the process of removing the tree completely, because the cuts made to it have destabalised it - even if it were left there, it would have died on its own."