Detective Sergeant Jason von Tunk, of the Portland police crime investigation unit, said the armed robbery happened about 2.10pm on Monday, on the corner of Lalor Street and Bridgewater Road.

"Two people were having lunch at a picnic shelter at the Graham Husson Fauna Park reserve off Bridgewater Road, an elderly woman and a middle-aged man," he said.

"Two men approached them. One produced a handgun, described as being like a pistol, and made a demand and a threat.

"He then said 'I'll take that' and took the gold ring off the male victim."

The investigator said the victims were left traumatised.

"It's pretty brazen offending - broad daylight on the side of a pretty busy road," Detective Sergeant von Tunk said.

"The two men who approached were both wearing black hoodies and surgical masks. We don't have much in the way of descriptions.

"We are requesting that anyone who was in that vicinity about 2.15pm Monday and saw anything suspicious to contact the Portland police station (5522 1500) or Crime Stoppers on 1800 333 000."

Anyone living nearby with CCTV, or a passing motorist with a dash camera, is also asked to check their footage.

The fauna park is right on the corner Lawlor Street and Bridgewater Road, where there's a shelter shed for picnics.

"It's where the white kangaroos are. The elderly woman and middle-aged man are Portland residents and they were just having lunch," Detective Sergeant von Tunk said.

"The two offenders are of an unknown age. We know they left on foot but don't know if they got into a car nearby or just fled from that location on foot.

"The victims were traumatised and the gold men's ring is of high sentimental value.

"We are looking into whether the offenders and victims were known to each other. It's unusual offending on a cold and wet Monday afternoon in public."

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