Instead, the Sydney mother listened to her daughter sing for the first time in almost 30 years.

As conductor Timothy Sexton announced what was about to happen, a murmur spread through the Australian National University's Llewellyn Hall and people reached for tissues.

The strains of a lone violin would have been appropriate enough, but Kim Kelly had the added emotional weight of the Canberra Symphony Orchestra behind her as she launched into Somewhere Over the Rainbow.

"It was unbelievable and the most magical day I've ever had in my life," Ms Grimwood said, wiping her eyes. "It was just heaven."

She was delighted the first singing voice she had heard since 1987 was that of the young woman who "had always been there" for her.

"I was five when she went deaf and I had to grow up pretty quickly," Ms Kelly said, recounting how she translated conversations with bus drivers and learnt sign language to communicate with her mum.

For the Cochlear Australia representatives who had kept the plan secret for nearly two months, the event was a celebration of the advancements in hearing technology.

"We wanted to raise awareness, as one in six people has got hearing loss," said Ian Gerrard. "There are only 223 people in Canberra with cochlear implants."

He urged people with hearing loss to consider finding a solution, rather than living with side-effects like depression and difficulty working.

"This is what bringing hearing back can do to people," said Grant Foley, who helped set up the surprise and drove Ms Grimwood to the Matinee Magic session.

He recalled how he thought "I can fix that" when she told him how she had seen her daughter performing at school concerts and auditioning for shows but had not been able to hear her.

Ms Kelly wore a mini-microphone and the sound was streamed into her mother's processor, as if she was singing straight to her.

"It was very, very clear," Ms Grimwood said. "It really, really works."