Officers said the woman stole envelopes that had been ”placed on a table for a baptism” during the ceremony at a church on Sladen Street in Cranbourne on November 19.
“It is understood she then moved into a preparation area off the main part of the church and stole a number of items including keys, microphones, a bag, a cross and a set of walkie talkies,” Victoria Police said in a statement.
Police believe she is caucasian and aged between 40 and 50 years old.
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“At the time of the offence, she was wearing white shoes, black pants, a blue hat and a black top with white stripes on the sleeves,” police said.
Anyone who recognises the woman is urged to contact police.