The Kambah man fostered brothers Tanner and Theo six weeks ago when they were one month old, and Red Dog a fortnight ago, through Australian Working Dog Rescue.

When he and his partner left the Australian kelpie rescue dogs outside in their pen before work last Wednesday morning, they were apparently running around playfully.

"When I got home at about 4.10pm, I walked out to the back where they normally are all excited to see us and jumping around, and Red was lying dead still on the back deck," the carer said.

"I called her name and she didn't respond, and then I noticed the blue vomit and the blue faeces next to her and started to realise she'd been poisoned, and I went into shock and was panicked."

He found Theo around the corner, lying next to a near-empty snail bait box that had been ripped to shreds.

"I heard Theo whimpering and he was having little fits and seizures, so I scooped him up and put him back in the car and raced him down to the Monarch vet hospital and they were able to treat him there."

Red Dog and Tanner died of their injuries, but Theo survived because of the quick work of the foster carer and vets.

"The little terror is 100 per cent again. He's doing awesome," the foster carer said.

While thankful for Theo's recovery, he was unable to comprehend how anyone could be so cruel.

"Who wants to kill a 10-week-old puppy? They are the most innocent things in the world," he said.

"Red Dog was the sweetest dog you'd ever meet. She was just bundles of happiness all contained in that dog.

"We were looking forward to giving that opportunity to someone else to enjoy and some low-life decided it's not going to happen ... I hope the cops find that scumbag."

The man – who asked not to be named – had fostered working dogs for years, because he enjoyed saving them from the pound and helping them be placed in new homes "where they could have a better life".

Despite admitting he'll take a step back from fostering for a while, he's determined to continue making a difference in the lives of dogs.

"For now I'll help with other areas in the [Australian Working Dog Rescue] organisation with transport and the paperwork side of things for a while. But eventually I will definitely go back to fostering.

"It's such a rewarding thing to do to be able to pass on the happiness of a working little dog and the organisation, it's unreal.

"They've supported us through the ordeal and they have been like a family, so I'm definitely going to be a part of that for a long time."

A post on Australian Working Dog Rescue's Facebook page calling for witnesses was shared more than 8000 times.

Police are hoping to speak to a boy believed to be in his late teens who was spotted around the Tepper Circuit area around noon.

The boy is described as Caucasian in appearance, short brown hair and of a medium build.

He was wearing a brown or red T-shirt, dark long pants and carrying a dark backpack.

The crime carries the maximum penalty of $1000 or six months' imprisonment.

People with information about the incident are urged to call Crime Stoppers on 1800 333 000, or report via the website at act.crimestoppers.com.au using reference number 5877800.