"She was too weak to stand on her own, and was covered in fleas," the SPCA statement said.
A second vet found Bella to be emaciated, moderately dehydrated and had tartar build-up on her teeth.
Bella was also found to have a grade 4 heart murmur and excessively overgrown nails.
"Both vets agreed that Bella had been left untreated for a long time, causing her undue stress and suffering, and that it would have been obvious to any lay person," the statement said.
"Following the examination, Bella was humanely euthanised to prevent further suffering."
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SPCA chief executive Andrea Midgen said the case highlighted the failure of the owner to take care of their dog who suffered for many months with complicated health issues.
"This dog had complex medical needs and was never taken to a vet to alleviate her symptoms.
"As a result, Bella suffered for the last months of her life. It is the responsibility of all pet owners to seek veterinary care for their animals if they require it."
Bella's necropsy revealed she had significant skeletal muscle wasting and had been suffering from Cushing's disease, as well as congestive heart failure and pneumonia.
In an interview, the dog's owner said she had noticed Bella gradually losing weight in the previous six months and had given her more food and de-wormed her.
She noticed Bella looked "a lot worse" over the previous two months, with weight loss that was more "dramatic", but said she couldn't afford to take Bella to the vet and had been saving up to.
The owner was sentenced to six months' supervision, disqualified from owning dogs for five years, and ordered to pay over $1200 reparations to SPCA.