"It's just a matter of time before they are picked up. They can run but they can't hide forever," he said.
A search warrant executed by the SPCA found dog hairs in the boot of the couple's Toyota Corolla car which had been left in Kaikohe when the couple travelled south.
Mr Boyd said hairs from the car are being sent to ESR for matching against hair taken from the surviving pup and its mother.
The penalty for wilfully ill-treating animals is up to three years in jail and a fine of up to $50,000.
The dog - named Shy - has been taken on trial into a home in the Kaeo area, one of four offered to the SPCA after the dogs' plight became known.
The surviving puppy - named Wai Wai - is now about eight weeks old and has gone, also on trial, to friends of Mr Boyd's.
Meanwhile, Auckland SPCA is searching for a woman who they believe has information about one of the country's worst ever cases of animal neglect.
SPCA staff believe Paulette Taki, whose last known address was in Glen Innes, Auckland, can tell them why Eve the dog was locked under her owner's house and left to starve.
The "very special dog" was found on December 24 and named Eve by SPCA staff.
The dog had been locked up and deprived of food and water for at least five weeks.
SPCA Auckland general manager David Lloyd-Barker urged Miss Taki, or any member of the public who has details of her whereabouts, to contact the SPCA.
"We feel that it would be very useful in progressing this case into the treatment of Eve to speak to Miss Taki as soon as possible," said Mr Lloyd-Barker.
There were no excuses for Eve's treatment, he said.
"We are required by law to give people the opportunity to explain themselves and we'll do that.
"They may have a reason but I can't imagine what it will be," he said.
The 11-month-old ridgeback cross weighed just 10kg when she was found, every bone was visible and she could barely stand.
Since moving to the SPCA Eve has gained condition and many admirers.
She would stay with the SPCA until she reached a "functional weight" and then go into foster care during any court case before being adopted out.
- ADDITIONAL REPORTING NZPA