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

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

The man was arrested later on Thursday, Senior Sergeant Andrew McDonald said.

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 can call Constable Jason Wakeling on 021 191 5462 or Crimestoppers on 0800 555 111.