Mum, three children witness fatal stabbing on Brisbane's bayside.

The ex-husband of a Brisbane woman has allegedly stabbed her new partner to death in front of her kids, before turning the knife on himself.

Officers said neighbours called them to the Brompton Street home at Alexandra Hills on Brisbane’s bayside just before midnight.

Police said the 39-year-old ex-husband of a woman living at the home broke into the house and stabbed her 37-year-old new partner, before stabbing himself.

The 37-year-old man died at the scene, while the alleged attacker — the father of the children — was taken to the Princess Alexandra Hospital on Brisbane’s southside in a critical condition.

Ambulance officers said the woman and other occupants of the house were not injured.

Police said the woman had a protection order against the man.

Inspector Rob Graham said police would provide support for the woman and three young children, aged 2, 7 and 11.

“The 37-year-old male had started a new relationship with the 37-year-old female — the ex-wife of the accused killer,” he said.

“We’ll certainly refer [the woman and children] them to the right agencies,” he said.

“But whatever help that can be given to them, there’s still stuff that can’t be unseen, and they’ll carry that into their lifetime.”

Forensic officers are examining the property.

Neighbour Trish Kennedy said she cared for the children initially after the incident.

“Considering whatever they’ve seen, they were in a good state,” she said,

“[I] made them warm, gave them a drink and talked about other things, watched [a children’s TV program] about 1:30am.

“It’s the first time I’ve seen the kids, because [they’re] very quiet people … honestly the quietest neighbours — they would have to be quieter than us.

“Just normal kid noises coming out of there and normal dog barking and that’s about it.

“I just got the kids and that was it — so I truly don’t know what else happened up there.”

This post originally appeared on ABC News.

© 2016 Australian Broadcasting Corporation. All rights reserved. Read the ABC Disclaimer here