Story highlights Police say the mother of 7 of 8 slain children has been arrested

Eighth child was the woman's niece, police say

The woman is under guard at a local hospital

The children were between 18 months and 14 years, police say

The mother of seven of the eight children found dead in a house in the northeastern Australian city of Cairns has been arrested on suspicion of murder, police Detective Inspector Bruno Asnicar said Saturday morning

The 37-year-old woman is under police guard at a hospital and in stable condition, he said at a news conference.

The discovery of the bodies added to a traumatic week for Australia after a deadly hostage-taking in Sydney.

Police said they were called to the suburban house in the Manoora area of Cairns on Friday morning after reports of an injured woman.

Inside, officers found the bodies of eight children between the ages of 18 months and 14 years, police said.

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Photos: 'Unspeakable crime' in Cairns, Australia Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – A police officer works at the scene where eight children were found dead in a suburban home in Cairns, Australia, on Friday, December 19. Hide Caption 1 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – Members of the public pay their respects at a makeshift memorial across the street from the scene of the crime. "The news out of Cairns is heartbreaking," Australian Prime Minister Tony Abbott said in a statement. "All parents would feel a gut-wrenching sadness at what has happened. This is an unspeakable crime." Hide Caption 2 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – A woman reacts at the scene of the crime. Hide Caption 3 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – Locals sit on the sidewalk near the house as police continue their investigation. Hide Caption 4 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – A woman lays flowers across the road from the crime scene. Hide Caption 5 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – Police work at the house where the bodies were found. Hide Caption 6 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – Police work at the scene of the crime. Hide Caption 7 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – People grieve near the scene of the crime. Hide Caption 8 of 9 Photos: 'Unspeakable crime' in Cairns, Australia 'Unspeakable crime' in Cairns, Australia – Police block the road near the house. Hide Caption 9 of 9

The eighth child killed was a niece of the woman, Asnicar said.

"They're only babies," Lisa Thaiday, a relative of the family, told CNN affiliate Seven Network. She said the horrendous scene had been discovered by a 20-year-old brother of the children.

"He got off the bus and he found his mother and siblings," she said.

Police said Saturday they talked to the 20-year-old but that he is not a suspect.

Seven Network reported that the children were part of the local community of Torres Strait Islanders, an indigenous people from the region.

Australian media reported that at least some of the victims were believed to have been stabbed. But Asnicar declined to confirm that, saying police are waiting for autopsy reports.

'Trying days for our country'

Images from the scene showed distraught neighbors gathered near the area cordoned off around the one-story house, where Christmas decorations were still hanging in the front window.

"It is so difficult to come to terms with what has happened on a street I have visited so many times among residents I have so often spoken with," said Gavin King , the local member of parliament.

Cairns is a remote coastal city of roughly 150,000 residents that's popular with tourists as a jumping-off point for visiting the Great Barrier Reef.

"The news out of Cairns is heartbreaking," Prime Minister Tony Abbott said in a statement. "All parents would feel a gut-wrenching sadness at what has happened. This is an unspeakable crime."

Australia had already been shaken this week by a dramatic siege at a cafe in the heart of Sydney, in which two of the hostages were killed along with the gunman who held them captive.

"These are trying days for our country," Abbott said. "Tonight, there will be tears and prayers across our country for these children."

'They never annoyed us'

The road where the house is situated has been blocked off and the crime scene locked down while forensic experts carry out their work, police said.

Specialist police officers will also be arriving from Brisbane, another city in Queensland, Asnicar said.

"This is not a small job. We're not taking this lightly at all," he told reporters. "We will cover every angle before this is finished."

A neighbor of the house where the bodies were found said the family who lived there had always been very quiet. She told CNN affiliate Seven Network that she'd heard no noise from the house Friday as she sat in her front room -- until the sudden burst of police sirens.

Another neighbor, Patricia Birch, said she heard some arguments on the property around 10 a.m., but she agreed the family wasn't known for causing problems.

"They never annoyed us," she told The Cairns Post. "They were really good."