Maia woman Jessie Tamara said she lost her cat, Bear, early last week.
He was missing for about five days until Friday, when Ravensbourne firefighters were called to a nearby manhole on a construction site which another woman's cat fell into.
"Her cat was down in the same hole where my cat was. She was able to save hers, but ours was just trapped and drowned because it was full of water.
"It could have been prevented."
Her 19-month-old son was very attached to the cat.
"You can't mention his name around him because he'll start looking around. It's really sad."
She described the cat as affectionate and friendly.
Blackhouse Properties Ltd director Dan Weir said he and his team sent their condolences to the cat's owners.
"This was a mistake on our part and we deeply apologise.
"Unfortunately, we understand nothing will bring back their beloved companion but we can assure we have learnt from this incident and have measures in place to make sure this won't happen again."
The pipe was "bunged" before the incident, but the bung was found at the bottom of the pipe the day after the incident.
"We believe the bung blew out due to a build-up of water days before, but unfortunately this was overlooked."
He believed the cat entered a drain pipe in a manhole it recently installed as part of a subdivision development stormwater upgrade.
"After speaking with the SPCA they believed it was likely the cats may have been chasing each other, or one was in the pipe first and the other entered out of curiosity."
The site was well fenced with the usual safety measures in place, but unfortunately the team overlooked the possibility of animals entering the drain pipe, he said.
Dunedin City Council building solutions manager Paul Henderson said its main issue was the entrance to the subdivision site in Fern Rd was unfenced and could be a hazard for children.
An inspector visited on Saturday morning and contacted the owner, requesting the entranceway be gated when workers were not on site to prevent children entering.
The work done the same day by the developer.
