Police experts say if they had known about a dislodged manhole cover there would have been a better search of a drain where a toddler's body was found a week after she went missing.
An inquest into the 2009 death of West Auckland two-year-old Aisling Symes began on Wednesday.
Her body was found after an extensive search in which police considered the possibility that she had been abducted.
The inquest was told the first constable on the scene, Gareth Needham, searched the drain under the partly-opened manhole near the girl's house, but failed to include the information in his handover report.
A senior detective and a search and rescue specialist who later became involved in the search say that was an error, and if they had known about the cover it would have rung alarm bells.
Mr Needham told the inquest he saw the displaced manhole cover, but thought it was impossible that Aisling could have fallen in because the cover was heavy and the gap was a maximum of 12 centimetres.
Mr Needham says he searched the drain with his torch, then covered the hole for safety reasons. When questioned by the coroner, Mr Needham conceded he should have reported the drain to his senior sergeant.
Detective Sergeant James Watson told the inquest that Aisling most likely stood on the partly-opened cover, which then flipped her into the drain head-first.
Mr Watson says the toddler would have not been able to get out and been quickly covered with water.
Aisling's parents walked of out the inquest during that evidence.
Residents are expected to give evidence about complaints to the council about the manhole.
