"These animals were in pain. They were suffering."
Elizabeth Marshall, who had worked as a part-time volunteer at the centre for about two years, resigned after witnessing the gathering together of the doomed cats.
Mr Warren said yesterday there were 12 cats and six kittens now at the centre, of about 50 that had been housed there.
Since last week a single semi-feral kitten had been put down, he said, for "temperamental reasons" that included extreme timidity and nervousness.
He said five other cats from the centre had been rehomed last week and new protocols would be used regarding the acceptance, quarantine and management of sick animals.
Wellington barrister and SPCA volunteer Wendy Aldred late last week demanded the society launch an immediate investigation into the cat killing at the Masterton centre, revealed by the Wairarapa Times-Age.
SPCA national president Bob Kerridge, in an email response to Mrs Aldred, said he had "seen this disturbing article and will be seeking some immediate answers". He said Alan Wilson, from the SPCA national manager inspectorate and centre support, was leading inquiries into the killings.
Mr Wilson said yesterday the Wairarapa SPCA was an autonomous organisation, governed through a committee that operates in line with the constitution of the Royal New Zealand SPCA.
His inquiries would focus on confirming the mercy killings were lawful, reasonable and "within over-arching national policies", and whether the reportedly rampant disease was due to previously negligent management or an unavoidable outbreak of illness.
"It could be they acted reasonably and did the right thing. Sometimes the number of cats coming through the door can overwhelm the staff, the committee and ultimately the capacity of the centre."
Mr Wilson confirmed that Mr Warren, while serving as a regional chief inspector in 2011, had been investigated after complaints about his use of a captive bolt gun to kill a 6-month-old stray cat called Janie at Horowhenua SPCA.
SPCA chief executive Robyn Kippenberger told the Herald on Sunday an investigation had revealed that Janie was a sick colony cat with runny eyes, ripped ears and the feline immunodeficiency virus. She said Mr Warren acted within his legal rights to shoot the cat.