A heartbroken family has opened up about the rare and incurable neurodegenerative disease that will cruelly claim their baby girl's life before her sixth birthday.

Sarah Hutchesson-Webb and her fiancé Jake Powell's world was recently turned upside down when their 16-month-old daughter Mia was diagnosed with Tay-Sachs disease, a rare genetic disorder that affects the nervous system.

The terminal condition is caused by the lack of a vital enzyme which helps to break down fatty substances.

Without the vital enzymes, the fatty lipid builds up to toxic levels within the central nervous system which leads to the progressive death of nerve cells.

Tay-Sachs disease has no cure and a life expectancy of four to five years. 

Mia's family are now focused on giving their baby girl the best quality of life possible and bravely shared their harrowing story to raise awareness about the 'horrible' disease.

'It's a diagnosis I don't wish on anyone,' Ms Hutchesson-Webb told Daily Mail Australia.

'All your hopes and dreams for your child go out the window.

'We're still trying to wrap our heads around and come to terms with the diagnosis.'

The Mount Gambier couple from South Australia first noticed a regression in Mia’s motor functions six months ago and have since seen a myriad of specialists trying to work out what was wrong.

They were left shocked and devastated when a MRI confirmed their worst fears earlier this month.

'Our hearts dropped in a state of disbelief,' Ms Hutchesson-Webb recalled.

'We could tell something wasn't right but we went along thinking she has something that would be curable.

'I burst into tears when I researched it on the internet afterwards.

'The hardest thing is knowing my only daughter will never walk down the aisle on her wedding day.'

Telling their young sons Hunter, five and Aston 2, was also difficult.

'Hunter knows his baby sister is very sick but Aston is too young to understand,' Ms Hutchesson-Webb said.

The mum-of-three described her youngest child as a sweet little girl.

'If she had the opportunity to grow up, she would have turned into a gentle and loving soul,' she said.

Mia's parents now focused on treating her symptoms to prolong her life and making her as comfortable as possible.

'The goal is to create beautiful memories with Mia and her older brothers and keeping her as healthy as we can,' Ms Hutchesson-Webb told Daily Mail Australia.

'We don't know how much time she has left.'

'Dealing with Mia's diagnosis drives home how much more we appreciate the little things in life.

'We are also desperately trying to create awareness around this horrible disease that most have never heard of.

'Out of all the people I have spoken to about Tah-Sachs since Mia's diagnosis, only two have ever heard of the disease.'

The close-knit Mount Gambier community has rallied around the family as they prepare for the harrowing journey ahead.

Mount Gambier Golf Club, where Mr Powell works as a greenkeeper, hopes to raise $15,000 at a fundraiser for the family this Friday night (February 17).

Organisers expect 400 locals to attend while businesses have pitched in by donating dozens of auction prizes. 

'We're speechless and very grateful of the incredible love and compassion shown towards us,' Ms Hutchesson-Webb said.

Friends have also set up an online fundraiser.

'For those who don't know, their beautiful little daughter Mia has been diagnosed with an incurable disease,' the page states.

'Tay-Sachs eventually leads to blindness, deafness, and paralysis. Children with infantile TS usually pass away before 5 years of age. There is currently no cure.'