Warrnambool City Council tree management staff first discovered drill holes in the 80 to 100-year-old tree on Nicholson Street six months ago and have since found fresh holes near the base of the trunk.

The vast tree is about 30 metres high.

Warrnambool Botanic Gardens and Trees team leader John Sheely said he had never seen vandalism to a Norfolk pine, a tree so integral to the city, before.

"It's an iconic avenue that relies on that uniformity of planting," he said.

"It's really disappointing to see such a significant tree willfully damaged.

"An astronomical amount of holes have been drilled into the trunk, 20 at least.

"Once the poison is injected there's nothing we can do, we just hope it can survive."

Two neighbours said they first noticed holes in the tree about a year ago but haven't heard or seen anything suspicious.

"It appears they'd tried to cover up the holes with pine needles and branches," one said.

Council Infrastructure Service manager Luke Coughlan said tree vandalism has increased over the COVID-19 lockdown period.

"Even illegal lopping has increased," he said. "We spend a lot of time and money managing street trees, to lose a significant tree will be a real loss of heritage and waste of resources.

"We will continue to monitor the health of the tree, it's got a big battle ahead to survive.

"We ask residents if they know anything or have seen anything to please come forward so we can take it to police."

He said it could cost upwards of $5000 to cut down if it completely dies.

There were mass plantings of Norfolk Island pine trees in Warrnambool in the late 19th and early 20th centuries.

Warrnambool and District Historical Society researcher and president Janet Macdonald said the trees were an important part of the city's history.

"They've got quite a history in Warrnambool and had a lot to do with the establishment of the botanic gardens in 1866," she said.

"Ferdinand von Mueller, a government botanist and first director of the Royal Botanic Gardens in Melbourne, and William Guilfoyle who designed Warrnambool's Botanic Gardens - both were very keen on Norfolk Island pines for their grandeur and ability to endure the coastal climate.

"The town council of the day regularly sought advice from both Guilfoyle and Mueller not only on street plantings, but layouts for parks, advice on how to prevent coastal erosion which was becoming a huge problem following a few short years of European settlement. Cattle, horses and sheep had eaten or destroyed the natural vegetation which covered the sand dunes, so the sand regularly blew into the town, blocking roads and destroying farmland.

"It's a terrible pity because big trees that age are extremely valuable, it's hard to estimate the value of individual big trees of that size but thousands and thousand of hours have gone into them and thousands of dollars-worth. It's a criminal act.

"The canopy of trees are extremely important and are part of the vista of the town, especially from a distance. Look out any window in town and you'll see those big trees.

"They are gradually disappearing so it's important we protect the ones we've got."

Council is urging anyone with information to get in touch.

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