Six month old baby's 'horrific' death will haunt police

St John WA said multiple crews were sent on priority one to the address and a woman in her 30s was taken to Sir Charles Gairdner Hospital while a female teenager was taken to Perth Children's Hospital.

WA Police Commissioner Col Blanch said the death is being treated as a family violence incident.

"There would be very few cases where you would have a crime scene of such a horrific nature for our officers and all first responders involved," Mr Blanch told reporters on Tuesday.

"And certainly those officers would be entitled to their critical incident leave, but that's something that will most likely haunt them for the rest of their careers."

The investigation remains in preliminary stages and family members of the infant are assisting detectives with their inquiries, WA police said.