Whanganui police have dealt with at least nine aggravated robberies in the city since last December.
The file has been closed on one case but police are continuing to investigate three robberies, with arrests made in relation to five other incidents.
Investigations are ongoing into the aggravated robberies of the Rangitikei St dairy and the Z service station in Dublin St in September.
Four males aged 18 to 20 years robbed the Rangitikei St dairy on September 9. Three entered the shop, leaving one person outside. They used a hammer to bash the male owner and a woman who came to his aid, leaving both with head injuries. The robbers demanded cash and cigarettes but fled with only confectionery.
On September 22 two males, one armed with a knife, took tobacco and cash from the Z service station in Dublin St. Last week police released CCTV images of the offenders and appealed for help from the public to identify them.