'It's every mum's worst fear - what will happen when I die?'

"Night night", Rachel said tenderly to her baby boy, Jason, as she gently placed him in his cot.

"Nigh nigh", he would cutely utter back to her as they exchanged loving glances.

The Brisbane mum's evening routine didn't change as her son grew  — except for one heartbreaking omission.

"He was around two when he started not saying 'nigh nigh' to me anymore," she said.

That was the then-19-year-old single mum's first sign of the lifelong struggle she would be set to endure.

As a newborn, nothing was out of the ordinary with Jason. However, by the time he was almost three years old, he was diagnosed with global developmental delay, intellectual impairment and autism level 3 — and not long after, regressed to become completely non-verbal.

Watch: British charity Autistica is urging people to #understandmore about life with Autism Spectrum Disorder. Post continues after video.

Jason is now 18, and his mum can't remember a time when she wasn't anxious about whether her son was in danger — from himself.

"Ever since he was little, he's screamed and punched himself with both fists at once," she explained.

"He'll be like that at 3am, running into walls and making holes in them. Or he'll bite his knees and arms. The worst is these 72-hour meltdowns where he'll scream and punch himself and other things non-stop. All I can do is call an ambulance."

But when they've sought help during countless hospital visits, Rachel has been left hopeless.

"I've had to force the hospital to do brain scans on him because they say it's all superficial harm that he's doing to himself," she said.

"How can him punching himself in the head countless times not be doing him any damage?"

Rachel is also constantly frightened that Jason's erratic behaviour will cause irreparable harm.

Rachel with her kids, Jason and Sunday. Image: Supplied.

"Jason's obsessed with cooking, and over the years I've woken up to fires starting in the kitchen," she said worriedly.

"I have to have a fire extinguisher there all the time and I have protection from things now. He also plays with the taps and likes to keep the water running. I've called DOCS and the NDIS saying, 'He's not getting the support he needs. I can't watch him 24/7.

"How do I sleep when I wake up to a fire or the house is flooding?'"

What keeps Rachel on edge the most, however, is the ever-present risk of Jason never returning home.

"The scariest is when he absconds," she said.

"Last year, my partner took him to Woolies, and Jason decided he would rather go to Coles nearby, so he ran off in the middle of the supermarket, and he had to call the police to find him.

"We live on a six-foot-high fence that he can't climb up on, and I lock every door and keep the keys in a safe, and the key to that I would wear around my neck because I was so scared of not being able to get out of my house in a fire. He ran from the house when he was little too many times that I just have to live that way.

"For a long time, I would wake up every hour, freaking out about where Jason was because I was so wired to be worrying about whether he'd escaped or set the kitchen on fire."

With Rachel — who suffers from fibromyalgia — working as a disability support aide and caring for her toddler, she and her partner are stretched beyond their limits, so Jason resides with carers three nights per week (two of those being family members) while another carer stays in Rachel's home one night per week to assist and monitor him.

"Me giving up this much control and having other people take care of Jason is new, and I always worry about what others won't know about what Jason needs and likes that I do?" she said.

"No one is going to know him like I do, or care for him as much as me. Ever. We can communicate with each other without saying a word."

Rachel with her son, Jason. Image: Supplied.

When her grandparents passed away in recent years, Rachel came face to face with the frightening reality of her own mortality.

"Being there when my grandfather and grandmother died, I held their hands and thought about my own death one day. I asked myself, 'Who will be around my deathbed? Will I leave Jason alone? Who will take care of him?'," she said, holding back tears.

"I just want my child to have a comfortable, happy life. Is that too much to ask?"

With her fight for Jason's daily support through the NDIS being a constant weight on her mind, Rachel can't bear to think of what may happen after she can no longer go into battle on his behalf.

"He'll never be able to live alone and will eventually go to SIL (supported independent living) so he can be functional in his own space," she said.

"He'll forever need a carer. But getting the help he needs is so much more complicated now that he's an adult."

The mum-of-two even knows how she will be farewelled to ensure her son grasps the full extent of what has happened.

"I need to make sure I have an open coffin and make sure Jason sees me in it, and he sees that box go in the ground so that he understands exactly where I've gone," she said.

"I don't want him to be confused for the rest of his life about where I've gone. He needs to know 100 percent."

Rachel with her son, Jason. Image: Supplied.

Having benefited from a course herself, Rachel is now an advocate for the Stepping Stones Triple P Online program, which is free due to support from the Australian Government Department of Health and Aged Care under the Parenting Education and Support Program.

The funding supports free online courses from the Triple P – Positive Parenting Program for all parents and carers in Australia who have children under 12 years.

Read more:

• An open letter to the parents of autistic children, from Hugh Van Cuylenburg.

• 'At 48 I was diagnosed with autism. It made me reassess my whole childhood.'

• 'My six-year-old son Lenny has autism. I see it as his superpower.'

Feature image: Supplied.