Dianne was 5 months pregnant when she was attacked in a Perth carpark. Yesterday, she died in hospital.

WARNING: Aboriginal and Torres Strait Islander readers are advised that the following article contains descriptions of people who have died.

Dianne Miller was a week away from finding out the sex of her unborn baby when she was 'senselessly' attacked in a Perth carpark on Tuesday.

The 30-year-old Noongar woman, who was five months pregnant, had reportedly stopped to pick up dinner with her partner and three nieces at Waterford Plaza Shopping Centre carpark when a concrete block was thrown through the open window of her car, 9News reports. 

According to police, an altercation had allegedly arisen between several teenagers and her teenager, who had been driving the car. 

"She didn't even see it coming. It hit her straight in the temple," her brother, Malcolm Clifton, told 9News. 

Dianne, who is also the mother of an eight-month-old son, suffered a heart attack and was rushed to hospital after members of the public attempted to resuscitate her. 

After remaining in a critical condition in intensive care, it's now been confirmed the 30-year-old and her unborn baby have died. 

Dianne's brother confirmed the news to NITV on Thursday minutes after she passed, saying their family have been left "destroyed".

Image: 9News.

"We just lost our world... the baby passed away too," he told the publication.

Her family told media the swelling on her brain could not be contained.

Malcolm described his sister as "one of a kind".

"Kind to everybody; good-hearted; good morals; well-respected."

Speaking to ABC, Malcolm said his sister had been through a lot in her life. 

"She just got on her feet, she just got a house, everything was working out for her and now everything is gone."

Relatives and friends of Dianne's attended a vigil outside the hospital on Thursday night.

Dianne's brother. Image: 9News.

 A 17-year-old boy who allegedly threw the concrete block faced Perth Children's Court on Wednesday charged with grievous bodily harm and breach of bail. He was remanded in custody and is due to return to court on December 9.

The parties involved are not believed to have been known to each other.

On Thursday, WA Police said homicide squad detectives had taken carriage of the investigation and made a fresh appeal for public assistance.

"This is, quite simply, senseless violence by a bunch of teenagers," Inspector Brett Baddock told reporters.

"From what we can gather, it's unprovoked. I think they've gone out looking for trouble and they've found it."

Premier Mark McGowan said it was a terrible situation for the family.

"It's very, very sad for her and her unborn child. It's one of those things that is just beyond description, that this could happen to someone," he said.

- With AAP.