The woman, believed to be in her 70s, died on Tuesday after being taken by ambulance to the Busselton Health Campus emergency department.
She had reportedly waited several hours to be seen after presenting with back pain and was monitored by paramedics while in the ED.
Health Minister Amber-Jade Sanderson on Thursday could not confirm whether the woman's care had been handed over to hospital staff when she had the heart attack.
She said the woman had been seen by staff, although it was unclear whether this was a doctor or nurse.
The death is being treated by the WA Country Health Service as a critical incident and a root cause analysis will be completed by independent clinicians.
"I don't have the exact timeframe (for how long she waited). All of that will be stepped through as part of the investigation," Ms Sanderson told reporters.
"I'm not prepared to draw any conclusions ... until we have all of the evidence around what occurred, including some of the clinical evidence which needs to be stepped through very carefully."
The woman had been mobile and went to the toilet about an hour into her wait, Ms Sanderson said.
An autopsy will be conducted to establish the cause of her death, while the root cause investigation could take up to six weeks to complete.
Ambulance ramping, where patients have to wait because emergency departments are not ready to receive them, has been a major issue in WA over the past two years.
The ramping hours for March, published by St John Ambulance, were the second-worst on record.
Ms Sanderson said there were complex issues including long-stay hospital patients waiting for NDIS or aged care placements.
She said initial reports suggested the Busselton hospital had been busy but fully staffed.
"It's busy in the south west. I think half of Perth is in the south west for school holidays, so it was busy at that time," Ms Sanderson said.
Opposition health spokeswoman Libby Mettam said the McGowan government had not done enough to address ambulance ramping and patient delays.
"There is simply no excuse for the health system to be so under resourced and mismanaged while the state boasts of multibillion dollar surpluses," she said.
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