The Fitzherbert Avenue Dairy in Springvale has been targeted by robbers for the second time in slightly more than two months.
Three masked men walked into the dairy about 7.30pm on Monday and demanded money and cigarettes.
Detective Senior Sergeant Dave Kirby, of the Whanganui CIB, said one of the robbers brandished a knife at the two store staff before the trio made off with a "reasonable amount" of money and cigarettes.
Mr Kirby said police knew the offenders were seen outside the dairy before the robbery.
"We believe they made off in a vehicle after the robbery but we don't have any description of the vehicle at this stage," he said.
The dairy's co-owner, who also owns another shop in the city, told the Chronicle that the three people came into the dairy demanding money. He said police were viewing footage from the shop's security camera.
The same shop was robbed on June 6 when a man went into the store and left with about $2000 in cash and several packets of tobacco.
Mr Kirby said anyone who knows anything about this robbery or who may have seen three people outside the dairy around 7.30pm on Monday should contact Detective Chay McArthur, phone 349 2827.