Police are investigating the unexplained deaths of a woman's two babies.

The six-week-old boy and his two-month-old sister died 14 months apart at a Canterbury property their parents rented.

The circumstances surrounding their deaths were similar, police said. Neither had suffered identifiable injuries.

In both cases, their mother, now aged 21, had told police she put them to bed in the morning after they had been fed. A short time later she went to check on them and found they had stopped breathing.

A team of detectives have been investigating since the baby girl's death on May 15. They have carried out a scene examination at the property and formally interviewed the children's mother and her partner.

Both denied any wrongdoing, police said.

Detective Senior Sergeant Greg Cottam, the officer in charge of the investigation, said police held concerns about the circumstances surrounding the children's deaths. Both were healthy in the hours before they stopped breathing.

As part of their inquiry, investigators had looked at a disorder known as Munchausen syndrome by proxy, a form of child abuse where a person harms someone to garner sympathy or attention from others, Cottam said.

Test results, which will identify any genetic factors that might have contributed to the children's deaths, could be several months away.

The woman no longer lives in the home where her babies died. She declined to comment when contacted.

"I've got a lot to say but I don't think it would be appropriate to go in the newspaper," she said.

Neighbours said the woman kept to herself and appeared isolated. She was unable to drive a car and rarely left the property, they said.

They believed she was a good mother.

A trained nurse who lives in the street said she was working in her garden the day the baby girl stopped breathing.

"I could hear her [the child's mother] screaming 'help' as she ran across the road. I could tell by the tone in her voice something was terribly wrong."

When the nurse opened the gate to her home the woman was standing there with her daughter in her arms.

"[The child] was like a porcelain doll . . . lifeless."

The nurse said she grabbed the child from the woman and performed CPR in the bedroom of her home until paramedics arrived.

The baby girl died in Christchurch Hospital the next day.

The woman's six-week-old son stopped breathing at the home on March 6 last year.

Paramedics were called, but were unable to revive him, Cottam said.

Another neighbour said the rental property where the woman was living was very cold and a drug user had lived there in the past.

Cottam said forensic testing revealed nothing in the home that would have contributed to the children's deaths.

The woman had voluntarily given up custody of her four-year-old son. Child Youth and Family (CYF) was working with the family to ensure the safety of that child, Cottam said.

"Without prejudging the circumstances surrounding the untimely death of these two infants, it is a reminder that our children are very precious and need all the love and protection they can get. Unfortunately police and CYF end up investigating far too many cases of child abuse and child deaths and it is important that anyone having concerns speak up to protect our most vulnerable."

He urged anyone with information or wanting assistance to contact Christchurch police on (03) 3637400 or Child Youth and Family on 0508 326 459. Information can also be provided anonymously by calling Crimestoppers on 0800 555 111.

What is Munchausen syndrome by proxy?

Munchausen syndrome by proxy is a rare disorder that involves a parent inducing an illness in a child they are caring for, often to attract attention.

Auckland psychologist Rebecca Daly-Peoples said people suffering from the syndrome were difficult to identify because they often presented as perfectly functional people.

"It's a cry for help. Rather than someone harming themselves, they harm their children and then they take them to hospital.

"Fundamentally it's a psychological disorder and the individual is needing psychological assistance."

It appeared cases rarely ended in death, Daly-People said.

"It's more frequent hospitalisations and reports of injury."

In 2013, a court heard how Auckland mother Shayna-Lee Broadbent, 20, who repeatedly suffocated then saved her newborn baby and then cried out for help, probably had the condition.

Broadbent pleaded guilty to causing grievous bodily harm with reckless disregard and was sentenced in the High Court at Auckland to 11 months home detention.

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