The service was led by parish priest Father John McHugh, who said Heath had been an "extraordinary young man".

Heath was only 15 years of age when he died on Saturday - but, Fr McHugh said, "It's not the length of the life that matters, it's how that life is lived."

Despite the many challenges the young man faced in life, Fr McHugh said Heath's "very large and loving family ... always made sure that [he] didn't miss out on anything".

"Heath knew that he was deeply loved and cared for by a family that only wanted the best for this special son and brother," he said.

"Heath, in his own exceptional way, touched and inspired the lives of many people. He inspired us with his courage and determination to get on with life.

"His infectious smile captivated us all. His love for life was plain to see."

The parish priest said Heath had been a "gentle soul" and had "brought so much joy and happiness" into the lives of his family.

"Heath had gifts that he shared with others such as the gift of generosity, the gift of laughter, the gift of wonder, the gift of gentleness, the gift of curiosity - and, of course, the gift of encouragement," Fr McHugh said.

The euology was shared by family friend Tony Blake, who wore a "light-hearted tie" with yellow ducks on it, bringing back "memories of Heath and the kids singing Rubber Duckie at a concert once".

Heath Archer King was born on July 28, 2003, to Bill and Katrina. He was a grandson for Rosemary and Henry Constable and a brother for Maddi, Lachlan, Oscar, Ryan, AJ, and Doug, who died tragically at the age of two.

Mr Blake said Heath was raised in a "very hectic household" and, despite running their own business, Mr and Mrs King were "very hands-on" parents who "found time to be actively involved in all of their children's activities". The Constables were "the bricks behind the family" and were always around to help.

At one year of age, Heath was diagnosed with Williams syndrome, commonly known as the cocktail syndrome.

"Children with this condition are almost always happy, easygoing, friendly, gregarious and love music, and Heath had all of these traits in abundance," Mr Blake said.

"Unfortunately, sufferers of Williams syndrome also have heart issues, which takes them away from us at far too young an age."

Mr Blake said that, despite the condition, Heath had a "joyful exuberance for life" and "threw himself into everything he did", packing a "huge range of activities and achievements into his short life".

"He would have an enthusiastic go at anything and everything, and was never daunted by the possibility that he may not be able to do it," Mr Blake said.

"If he could not do it, he never became despondent or upset - he would simply accept it just move onto something else."

Heath gained the nickname Kingo in daycare, and carried this with him when he attended Carinya Christian School and, later, GS Kidd Memorial School, where he was enrolled when he died.

"His zest for life, friendly and easygoing manner, his enthusiasm and willingness to participate endeared him not only to his fellow students but to all the teachers and volunteer staff and other parents," Mr Blake said.

Mr Blake said he had "pages" of notes from GS Kidd staff and students about Heath, which were "too numerous to read but all pay testament to Heath's many wonderful characteristics".

"Their thoughts are best summed up in a very profound statement by one of his classmates ... 'Legends never die but, when they do, they will live on in people's hearts'," Mr Blake said.

Heath enjoyed watching roadworks and visiting Bunnings, and he loved all things mechanical.

Mr Blake recalled a time when he was making cabinets in the Kings' shed and "there, looking over my shoulder, would be Heathy".

Heath also had a fascination with flying and "a great love" for all manner of sport.

"No matter what sport it was, he embraced it with the same enthusiasm as he did with everything else," Mr Blake said.

"He embraced all of his brothers' sports and was taken under the wings of all the cricket clubs, the league club and the rugby club - and anywhere there was a game on, Heathy just wanted to be there.

"He was an official member of the Courthouse Cricket Club and an unofficial member of all the other clubs."

However, Mr Blake said "by far Heath's greatest passion in life was the Gunnedah Shire Band" of which he was a "valuable member", along with his family.

"If the Gunnedah Band had a mascot, Heathy would be it," Mr Blake said.

"He was destined to play percussion in the band and follow in the footsteps of the rest of the family, [and] at band concerts, Heath would be seen standing up near the stage conducting in his own quaint way.

"He played the bass drum with enormous gusto and enthusiasm, always loud and frequently getting in a few extra beats after his part had ended.

"He took his role as junior drum major of the band very seriously, proudly marching in front of the band with a huge smile on his face, decked out in full band regalia, twirling his miniature mace."

Mr Blake recalled fondly a Brilliance of Brass concert in Tamworth, where Heath came forward at the end of the Gunnedah band's performance "with his arms raised in jubilation, and took a great bow in the centre of the stage as though the cheers and applause from the 600-strong audience was entirely for him".

Mr Blake said the death of Heath would "leave a huge void in the lives of so many people in Gunnedah and beyond".

"Heathie, you will remain much-loved in all of our hearts and memories, and you will be dearly missed by so many of the people whose lives you touched," he said.

"We never looked at his disability as what defined him, but rather what made him thrive ... What some might consider a hindrance, Heath instead showed to be a great strength and his most endearing quality.

"He had a deep capacity to love unconditionally and spread happiness effortlessly. He saw the world not for what it is but for what he wanted it to be."

"You'd go in the dressing sheds for footy and he'd always be in there giving this little speech - 'We're going to smash these idiots'.

"It didn't matter if we got beat or not, we'd still smashed them in Heath's eyes.

"We'd run out onto the field and the only person you could hear from the stands was Heathie ... 'Get off him! He's my brother, you idiot.' "

"Heath had to fight hard to be part of life and made sure he lived every day, even though it meant months in hospital and hundreds of doctors' appointments. He very rarely complained - he just got on with it.

"He got up every day and Dad knew he'd make someone smile that day because of his kind nature, his zest for life and his genuine interest in whoever he was with.

"Mum was the centre of his universe."

In lieu of flowers, the King family has asked the community to make donations through the My Cause website for the Heart Foundation, GS Kidd Memorial School, The Starlight Foundation and Westmead Children's Hospital.

To donate, visit https://www.mycause.com.au/page/206743/heathy-our-little-champion