Police are investigating an alleged assault at a popular Whanganui park on Tuesday.

The Bason Botanical Gardens on Rapanui Rd near Mowhanau were closed just after 2pm.

An officer was stationed at the front gate and was not allowing the public to enter.

"A male attempted to assault a female before she has been able to run off and raise the alarm," police said in a statement.

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Police were speaking to witnesses and the victim. "There's no further information available at this time."

The park reopened about an hour later.