"I watched her disintegrate." Mum recalls daughter's final plea in thunderstorm asthma.

On November 21 of last year, a catastrophic weather event changed the lives of nine families forever.

As five storms combined into one, thunder and rain battered Melbourne, bringing with it an unseen danger: exploded rye grass pollen formed into tiny particles small enough to enter people’s lungs and trigger devastating asthma symptoms.

The unprecedented event caused the death of nine people, including 20-year-old Hope Carnevali.

Speaking to 60 Minutes, Hope's mother, Danielle, recalled watching her daughter deteriorate before her eyes.

"It's indescribable, I'm watching my daughter disintegrate, barely able to move," Danielle said.

"It was traumatic."

The family called for an ambulance and were told one was on the way, but the family's anxious wait soon stretched to 30 minutes. As Hope's conditioned worsened, the family continued to make calls to triple-0.

Despite the fact a hospital was just a six minute drive away, the family was told to stay put and wait for help to arrive.

20-year-old Hope was at home when she began struggling for breath. Her family called for an ambulance & were told it had been dispatched. pic.twitter.com/fvtni7qvOq

— 60 Minutes Australia (@60Mins) May 14, 2017

"The hardest part for me as a mum is I had that chance of saving my daughter's life taken away from me," Danielle said.

"Had I been told that an ambulance wasn't in the vicinity, it wasn't dispatched, they had none left, I would have put her in that car in a heartbeat.

"I would have had her at that hospital, which is about six minutes away, and she would still be here with us today.

"I have to live with that guilt because she asked me to help her, and I didn't help her."

Danielle said her daughter started turning blue, and begged her mother for help before she took her final breath.

"[She said], 'Mum I'm dying, mum I can't breathe, mum help me'.

"To not be able to help her, as a parent that is the hardest thing you can ever go through."

Her devastated family attempted CPR on the front lawn of their own home as they waited, but Hope could not be revived.

The November 21, 2016 storm is the world's worst recorded thunderstorm asthma attack, and saw more than 8500 people hospitalised in Victoria.

"We intended to get to everyone possible - you call us, we will come," Ambulance Victoria's director of emergency management, Paul Holman, told 60 Minutes.

But the need was "unprecedented", with a call for help being made every four seconds during the storm.

"We had an unprecedented emergency, a major disaster. We will learn - how can we get it better next time?" he said.

Following the 60 Minutes story going to air, Asthma Australia has issued a statement to those suffering from asthma or experiencing hay fever.

"The thunderstorm asthma event was an unprecedented and tragic episode," Michele Goldman, CEO of Asthma Australia said.

"Proactive steps that can be taken to improve management of asthma or hay fever symptoms are to consult a GP, pharmacist or call 1800 ASTHMA”.

Asthma Australia is urging anyone concerned about thunderstorm asthma to consult their General Practitioner, pharmacist or call 1800 ASTHMA (278 462) for information and support.