Melissa’s mother, Lynda Tucker, said the family discovered it on Tuesday when they visited the cemetery after rain and hail the previous day, on what would have been Melissa’s birthday.
“I thought it was bulletproof… unfortunately it was brass,” she said.
The brass dolphin was removed from Avonhead Cemetery.
Tucker said she had contacted scrap metal dealers across the city, but did not believe the piece would be recovered.
“Unfortunately, [we] don’t know exactly when it went,” she said.
“We will have to look at some other substance to replace it with.”
The theft comes amid a string of incidents involving brass and bronze memorials across Christchurch.
Last month, plaques were taken from Latimer Square and the Bridge of Remembrance overnight on Tuesday, August 11, and Wednesday, August 12.
“Police are working to locate the plaques and establish what has happened to them,” Metro Commander Superintendent Lane Todd said at the time.
Last year, thieves also targeted a memorial commemorating victims of New Zealand’s deadliest fire.
Twenty-three of the 41 bronze plaques honouring those who died in the Ballantynes fire were missing when NZME visited Bromley’s Ruru Lawn Cemetery at the time.
One row of the memorial had been completely stripped by vandals.
At least seven plaques from wooden benches throughout the cemetery had also been targeted, with some benches damaged during the removal process.
The department store blaze remains the deadliest fire in New Zealand history, claiming 41 lives.
At least 680 people were inside Ballantynes when the fire broke out, including 430 staff and 250 customers.
Jazlyn Whales is a multimedia journalist based in the Christchurch newsroom.