People in Whanganui's Alexander Library thought there had been a car accident when the building suddenly shook; instead they discovered a massive branch had fallen from a tree outside.

Lynley Fowler was in the heritage library on Tuesday afternoon, January 9, when she heard "a rushing noise".

"Then there was a thump and then the building shook," Ms Fowler said.

"At first I thought there'd been an accident down on the corner but when I started heading out to have a look I saw the tree and you could smell it."

Advertisement

The branch belonged to a large macrocarpa which is protected in the Whanganui District Plan. It stands alongside the path that runs from Cameron Tce to Queens Park.

Whanganui District Council parks officer Claire Lilley said the age of the tree was unknown and it was thought it was protected because of its size and contribution to the landscape.

"They are weighty trees with long branches that carry a lot of weight at the end and a big macrocarpa comes with an element of risk," Ms Lilley said.

"Last week we had a lot of rain after a long period of dry and that can sometimes cause sudden branch drop."

Ms Lilley said the branch that fell formed a big percentage of the tree's crown. The tree was last inspected in April and showed no signs of rot.

The branch would be cleared and the stub tidied, she said. It was likely the height would be reduced to lessen the risk, making the tree more compact and less liable to wind damage.