Detective Senior Sergeant Dave Kirby, officer in charge of the Whanganui CIB, said he was disturbed by the "audacious nature" of the offender and what he did.
"But I'd give a strong reminder that the costs of this sort of robbery carries with it a maximum of 14 years in jail.
"In my view anyone convicted of something like this can expect to spend a reasonable length of time in prison.
"It surprises me the lengths these people will go to carrying out this sort of offence knowing what the consequences are."
Mr Kirby said Whanganui Police were still interested in the movements of the small white hatchback that was parked in St Leonard St.
"We believe the offender has visited the store several times in the days leading up to the robbery so someone may well have seen the vehicle in the area over the last week."
There was a "strong likelihood" the offender made substantial purchases of alcohol or cigarettes soon after the robbery.
"We're asking any retailers who has handled purchases of hundreds of dollars worth of alcohol or cigarettes to get in touch with us as well."
Mr Kirby said there are sure to be some in the Wanganui criminal fraternity who know about this incident and who could be involved.
"If they have information they can call me on my direct line (349 0648) and any information will treated confidentially.
For general information about the robbery contact Constable Paul Heathcote on 349 0618 or call 0800 Crimestoppers.