Yanchep Sergeant Jason Macander said two men used a ute to knock over the red mail box outside the Yanchep Central shopping centre early Monday morning.
Sgt Macander said the pair then loaded the post box onto the vehicle's tray and left.
An Australia Post spokeswoman said police informed them the mail box had been stolen on August 24.
"Our corporate security and investigations team is working closely with Yanchep Police on this investigation to recover any mail items in the street posting box at the time of the theft," she said.
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READ NOW"Australia Post takes all incidents of this nature very seriously and urges residents of Yanchep and surrounding suburbs to report suspicious behaviour around street posting boxes to local police.
"Residents who are concerned their mail may have been affected can contact our customer contact centre on 13 13 18."
A boy (11) found the box in nearby bushland on Monday afternoon and forensic police are investigating.