A Whanganui man has been arrested over an alleged assault in the car park at Pak'n Save.

The man was arrested about 3.15pm today, Senior Sergeant Andrew McDonald said.

Police allege the man was in a car with the complainant about 11.20am on Thursday and assaulted her.

The car was in the upper parking area outside the supermarket at the time.

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"A number of people witnessed the incident," Mr McDonald said.

Police are asking any witnesses to come forward with information.

Anyone with information is asked to contact Constable Jason Wakeling on 021 191 5462 or call Crimestoppers on 0800 555 111.