A member of the public discovered the body of the female dog - a tan mastiff-type cross breed about four years old - on Monday.
Animal welfare inspector Brett Lahman says the weight was discovered close by, with a dog's collar and lead still attached.
He says initial post-mortem results indicate that the dog most likely died from drowning and was possibly alive when it entered the water.
"A purposeful act has been carried out by somebody to cause an animal to suffer an extreme level of distress, so it's definitely on the higher end of offending," he says.
Mr Lahman says the SPCA is looking into the possibility the dog was stolen.