Whanganui Police Detective Sergeant David Thompson said police were called to the store at 4.52pm on Saturday after a male with a hammer demanded cash.
"An amount of cash was taken but no one was hurt," Thompson said.
"The offender left on a bicycle in the direction of Harrison St and then to the Glasgow St area."
Police have conducted a scene examination and are reviewing CCTV footage. They are appealing for sightings of a male, who was wearing a red hat, on a bicycle around 5pm on Sunday.
Anyone with information can call Detective Karl Reyland on 06 213 4180 or make an anonymous call to Crimestoppers on 0800 555 111.