Dog control was "getting to the bottom of it" and they had a good idea of who the dogs belonged to but could not yet confirm it, Mr Robinson said.
"We said the owners should be prosecuted ... because to be honest at 9 o'clock at night, if the kids had been out and the dogs had come on the property it could have been quite a different story.
"If they've got a history of it the dogs should be destroyed, if you can't contain your dogs and they do this kind of damage to livestock the chances of it happening to a child are very high.
"It's just not worth the risk."
Mr Robinson had put the only uninjured animal, a black lamb, into the laundry after the first attack, which meant it stayed safe when the dogs returned.
It was the only newborn lamb to survive.
"We think we might call him Houdini," said Mr Robinson.
"We're waiting to see if one of the ones who survived was its mother, we're not too sure yet."
If the mother of the lamb was dead, the lamb would need to be hand-raised.
A vet was coming at 12.30pm to check on the sheep who had been injured, Mr Robinson said.
Auckland Council animal control staff have contacted the dogs' owners and were holding one pit bull at an animal shelter in Wiri.
Animal control manager Tracey Moore said the second dog was still loose and asked locals to contact the council if they saw it.
"The second dog who has not yet returned home but the owner has relinquished the dog to us once we find it," she said.
"A full investigation is being carried out on the incident and we cannot comment further until this is completed."
The council reminded dog owners that all dogs must be under their control at all times.
It was also the owner's responsibility under the Act to ensure that their dogs don't injure, endanger or cause distress to any person, stock, poultry or domestic animal, or protected wildlife, the council said.
If anyone sees the dog they can contact Auckland Council on 09 301 0101.
