An Auckland family are in mourning after their assistance dog was killed by possum poison at an off-leash dog park.

Casey, a labrador-retriever cross, was celebrating her third birthday at the Waiata Reserve in Remuera when she found the fatal poison on September 13.

Assistance Dogs New Zealand (ADNZ) posted a statement from the Harris family - Casey looked after their young daughter.

SUPPLIED Casey enjoyed the outdoors.

The family, who did not want to be named further, said Casey, as usual, spent her time running through bushes – her favourite pastime in the free-run areas.

The following afternoon they noticed Casey was drinking a lot more water than usual, was urinating frequently, was vomiting and becoming increasingly weak.

They took Casey to the vet and she remained in veterinary care until she died on September 19.

SUPPLIED The possum poison that killed Casey was not two metres above the ground as it is meant to be in areas frequented by dogs and children.

Blood tests confirmed the product that killed Casey was cholecalciferol possum bait which is available at Mitre 10 or Placemakers.

As Casey was a working dog they were able to re-trace her footsteps. When the family followed Casey's paw-prints into the bushes at the park they found a bait station.

The family said the bait's instructions were not followed as "in an area frequented by dogs and/or children, this bait station should be placed at least two metres up the tree. That's about the height of your door!"

SUPPLIED The Harris family are grieving following the loss of Casey

The family said they were missing Casey a lot.

"Her absence is felt in every part of our day to day lives. From getting up in the morning, doing the shopping, driving the car, watching TV, reading books, doing homework, to going to sleep, the list goes on.

"We wish she were here to help us through."

SUPPLIED Casey helped the Harris family's youngest daughter.

The discovery was reported to the council who came to the family within hours of them reporting it. It was not a council bait station.

Auckland Council parks manager Mark Miller said the bait and station were not the kind used to manage pests in council reserves.

"Council contactors use secure containers, designed to be tamper-proof, to stop other animals accessing the bait."

He said while anyone could buy bait and use it on their own property, they should not be laying it in a public place.

"This is a very distressing situation for all concerned, and whilst the laying of the bait may have been well intentioned, people need to make sure any toxic bait is secure so that other animals cannot access it," Miller said.

ADNZ said the bait was easily purchased as an off-the-shelf product.

"Unfortunately there are no register or serial numbers on the product to track it.

"The investigation continues and hopefully the folks that put the station there will be advised of the consequences of their actions."

Assistance Dogs New Zealand

Chief executive and ADNZ trainer Julie Hancox said Casey was one of the first dogs the organisation had bred itself.

It cost $20,000 and a year-and-a-half to breed, raise and train a dog like Casey, she said.

Assistance dogs are different to guide dogs as they are purpose-bred and trained to assist a range of disabilities.

"This is a huge and devastating loss because we are such a small charity," she said.

ADNZ provides dogs for a number of different and complex disabilities including: Down syndrome, autism, diabetic alert, Fragile X, Alzheimer's, and other complex disabilities or combinations of disabilities.

Assistance Dogs New Zealand has a waiting list of more than 40 people.

"This is the first time we have lost a dog so young, we match about 8 to 10 dogs a year with those on our waiting list.

"90 percent of the dogs go to children and as they grow we upskill the dog to match the person's needs."

Assistance Dogs New Zealand is a not-for-profit organisation that relies on dog sponsorships and donations.

Casey was born to ADNZ dogs Bella and Danny and was sister to Chase, Chance, Cooper, Callie, Cleo, Connie, Caz and Codee.

ADNZ posted an earlier statement on behalf of Casey's family.

"Rest in peace our beloved Casey. You were the best and so much more than just our noble Assistance Dog," it said.

"We miss you terribly and will remember you always for being the sweet, fun loving girl you were.

"Wish your time with us could have been many, many years longer. See you in heaven one day darling dog. We will love you always."

*This article has been edited to remove reference to Bunnings as a source of products with cholecalciferol. Bunnings said it sells no product with cholecalciferol.