Earlier, Crown prosecutor Josh Shaw opened the case against the men and said the shooting came about after Mendoza believed his dog had been attacked by a dog belonging to his neighbour Rowan Hargreaves.
Mendoza and his friend went to Mr Hargreaves' 5ha property near Wellsford. Mendoza took with him a note which he got Mr Hargreaves' to sign. The note said the men could shoot the dogs and there would be "no come back''.
He said the men found two dogs in a van and shot both of them in the head, making two "reasonably clean kills''.
A witness will tell the court that "at this point the dogs erupted''.
The two men went to a cage where a further eight dogs were kept and began shooting from both sides, Mr Shaw said.
"There was nothing clean or structured about it.''
He said dogs cowered behind other dogs or part of the cage to try and take cover.
Mr Hargreaves could not watch and left but a friend stayed behind to see that the dogs were killed cleanly, Mr Shaw said.
"It was clear that they were not killed instantly. They suffered considerable stress as well as pain.''
Mr Shaw said the shots also put the two men at risk, as well as Mr Hargreaves' friend.
The two men then moved to a campervan where 21 puppies were kept.
He said there was no suggestion that the puppies had been involved on the attack on the Mendoza's dog.
"But for whatever reason, the accused were intent on killing every living dog in the campervan.''
He said they removed them from the campervan and put them on a concrete pad. One puppy escaped under the campervan and one of the men leaned under and took pot shots.
Mr Shaw said the whole shooting took up to 30 minutes and neighbours will describe hearing moaning and screaming from the dogs that sounded like nothing they had heard before.
"It seems for the accused it turned into something of a commando exercise ... after the first two dogs were killed, it really has been a case of killing every dog that was living.''
He said the two men had no concern for their safety and many of the dogs were not killed humanely. Some had multiple injuries.
"As Your Honour is aware, if an animal is put down, it needs to be done humanely and without the animal suffering unnecessarily pain or distress.''
Mendoza and Campbell face four charges of wilful ill-treatment of an animal and alternative charges of ill-treatment of an animal laid by the Society for the Prevention of Cruelty to Animals (SPCA) under the Animal Welfare Act.
The charges carry a maximum sentence of three years imprisonment. The men also face charges laid by police of reckless use of a firearm.
The trial, before a judge alone, is set down for two weeks.
