"That this tiny, innocent life should be taken based on (assumed) genealogy is totally unjust. A 4-5 week old puppy who has been offered numerous loving rescue and adoption opportunities should not be killed based on optional (BSL) Breed Specific Legislation," the post said.
The person who sent the post did not want to be named.
In a statement in response to Rotorua Daily Post questions, the council said it was taking a tough stance on its dog control policy and reinforcing the need for people to be responsible dog owners.
The statement included three graphic photos showing injuries suffered as a result of Rotorua people being attacked by pitbulls. The Rotorua Daily Post chose not to publish the photographs.
"The safety of the community is council's top priority and we make no apologies for that," council operations group manager Henry Weston said.
The statement said the council had been subject to "an aggressive campaign" to release the puppy, which was still in the council's care.
He said there was no evidence the female dog had had a litter, meaning it was unlikely to be the mother of the younger dog.
The female dog, a labrador cross, is due to be desexed and will be rehomed.
"Unfortunately, it is left to councils and their staff to deal with situations created by irresponsible dog owners," Weston said.
"The role of animal control services is to keep our community safe and we do that within national legislated guidelines and through council policies and bylaws.
"Nobody enjoys having to deal with these situations, least of all our staff, but they also see and deal first hand with the damage dogs can do."
Weston said the younger dog, which was still in the council's care, fell under the Menacing Dog Classification under schedule 4 of the Dog Control Act 1996.
He said it was the council's policy that such dogs would not be rehomed because of the risk they posed to the community.
"This has been in place since 2001 and is consistent with a number of other councils around the country."
He said animal control staff follow an evaluation checklist to identify types of dogs which came into the pound and in this instance the dog had been also identified as a pitbull by an independent vet.
He said while pitbulls made up just 3 per cent of Rotorua's registered dogs, more than a third of all dog attacks on people in Rotorua during 2017 were committed by pitbulls and so far this calendar year, 23 per cent of attacks were by pitbulls.
He said the number of attacks by pitbulls in Rotorua is three times higher than any other type of dog. Pitbulls are also responsible for a disproportionate number of attacks on other animals, with 16 per cent so far this year.
"While people may be upset at our decision now, people would be less impressed if we allowed the dog to be rehomed and there were issues down the track," Weston said.
Weston said the public campaign had also seen threats made to council staff and they intended to report it to police.
"Our staff are also members of the community, fathers, people with families, pet owners – the personal abuse they are experiencing from some of the more over-zealous commentators is unhelpful and uncalled for," Weston said.
"Sometimes we have to make difficult decisions which may be unpopular."
