Police are investigating the killing of a Titirangi family's guinea pigs which has left an 8-year-old boy distraught.

On Thursday morning the boy's solo mum went out to feed his two pet guinea pigs and noticed the cage had been broken into overnight.

The mesh sides were torn and the guinea pigs were missing - found several moments later with "clear knife strikes" on their bellies.

"There was like a line of blood on the belly and on the back."

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The bodies were stacked against the base of a tree, which also had knife marks on it.

The woman felt unnerved at the scene, in part because she hadn't heard anything out of the ordinary overnight.

She said she didn't understand why someone would do this, calling the incident "disgraceful".

"It was odd, very bizarre."

The woman, who asked not to be named to protect her privacy, said she was relieved she had been the one to go out in the morning and see the carnage.

After pocketing the dead animals so her son wouldn't see, she took them out to her mum's house and buried them.

She told her son the guinea pigs ran away rather than been killed but even so the loss had crushed him.

"They were like his babies, he's devastated."

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The woman had called the police, who confirmed they were investigating.

"This matter has been reported to us and inquiries are underway," said Detective Senior Sergeant Roger Small of Waitakere Police.

Police visited the woman's home yesterday afternoon and investigations were ongoing.

Anyone with information was asked to contact Waitakere Police on 09 839 0600.

Information could also be given anonymously to Crimestoppers on 0800 555 111.