Detective Constable Shaun Leo said he believed they could be linked because three robberies targeted a Cash Converters, and in all four jewellery was stolen.
That followed an offence on March 12 when two men held up the Cash Converters on Warnbro Sound Avenue in Port Kennedy.
He said these two incidents could be linked to armed robberies on a jewellery store on Yirrigan Drive in Mirrabooka on March 3 and a Cash Converters on Albany Highway in Cannington on March 4.
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READ NOWThe escape vehicles used in latest two robberies were found alight a short time after.
Four thefts have occurred in a two-month period, the most recent in Mandurah lastMonday about 9.45am at the Cash Converters store on Arnold Street.
One man entered the shop armed with a tomahawk and used it to smash a display cabinet, where he stole a large quantity of jewellery before running from the store and getting into a maroon Ford Falcon sedan.
The vehicle was discovered alight a short time later on nearby Anstruther Road.
On Thursday, March 12 about 3.55pm, two men held up the Cash Converters on Warnbro Sound Avenue in Port Kennedy, with one man holding open a sliding door while the other smashed a glass cabinet and stole a large quantity of jewellery. They ran from the store and left in a green 4WD, which was driven by a third person.
Det Const Leo said large amounts of jewellery were stolen on all occasions and two cars had been found burnt-out.
"We believe the same offenders have committed these robberies, certainly for the Mandurah one and in Port Kennedy," he said.
Anyone with information is asked to call Crime Stoppers on 1800 333 000.