The only documentation the cemetery had was an index card which listed Liadan simply as "Baby Rose".
Ms Innes said the family had been sure of where Liadan was buried because decorations on two adjacent plots had dates that matched Liadan's death.
But cemetery staff told Ms Innes the other plot-holders were "just guessing" where their babies were buried, and their plot decorations were not permitted.
Ms Innes said she and her mother were devastated.
"They give you a plot number, you believe that your child's going to be there and then you find out years later that they lied."
Auckland Council's acting regional and specialist parks manager, Richard Hollier, said he was sincerely sorry for any distress caused by the miscommunication. He said Waikumete Cemetery had records of all interments, as required under the Burial and Cremation Act - including for all 797 infants buried at the stillborn sanctuary since its establishment in 1951.
Baby Liadan's details were recorded on an index card in 1985, and had since been entered into a database which could be publicly searched online.