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SPCA Ashburton area manager Rebecca Dobson said it was a passerby who found the tiny bodies near the river last week and called the SPCA.
A post-mortem showed they had starved to death.
"They had little or no fat around their internal organs, and they weighed just 300g each.
They should have weighed double that.
"Dobson said despite public appeals for information, the culprit had not been found.
"But we have seen a really positive thing come out of it, that is lots of people posting on Facebook offers of help and support for others in need," she said.
"It is a really stressful time for everyone, with people losing their jobs and so much uncertainty.
"It can be really hard if you've got a lot of animals or an unwanted litter - or even if you've got really loved pets, it can be hard to feed them.
"That's why it's always really good if you can reach out - just reach out, we're always here to help.
"As an 'essential service', the SPCA's doors were always open," she said.
Pet food is included under "essential goods", for which financial assistance is available from Work and Income.
Meanwhile, reports of suspected animal neglect or abuse to the SPCA were down 25 percent during the lockdown.
Chief executive Andrea Midgen said it was feared animal suffering had been going on behind closed doors.
"We absolutely rely on the public to report and then we go and investigate.
"So the fact the public hasn't been reporting ... we're still trying to get underneath the reasons for that."
SPCA inspectors have continued to respond to emergency calls and urgent animal welfare complaints during the lockdown on a restricted basis.