Police are investigating the alleged sexual assault of a woman near Waikato Hospital early yesterday morning.

Detective Terri Wilson said they want to hear from any witnesses after a 31-year-old woman told police she was grabbed from behind by an unknown male, threatened and sexually assaulted.

The attack is alleged to have happened about 4.30am as the woman walked through the car park of St Stephen's Church, on the corner of Mahoe and Ohaupo roads, Wilson said.

"There is a possibility there were people in the area at the time who may have seen either the victim or the offender.

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"We really want to hear from anyone who may have information to help us."

Anyone with any information should call Detective Craig Chaplow at Hamilton Central Police Station on (07) 858 6200, or contact Crimestoppers anonymously on 0800 555 111.