Liam Danher's parents say a seizure mat could have saved his life, but the NDIS knocked back funding

Liam Danher died in Feburary. His parents say a specialised seizure matt could have saved him. (Supplied)

Parents Kevin and Tracey Danher have spent the past three months overcome by grief after the death of their 23-year-old son Liam.

But mixed with their grief is a strong feeling of injustice, as the couple believes the National Disability Insurance Agency could have taken steps to save their son.

Liam was severely intellectually impaired and had the mental age of a toddler. He also lived with autism and epilepsy.

On the morning of February 5 this year, the Danhers awoke in their Cairns home and checked on Liam, who had been sleeping in the next room.

He had died of a seizure in his sleep, his parents say in a letter to NDIS Minister Linda Reynolds.

"Normally he was up pretty early … We walked in and found him, and I just knew straight away. It was a big shock. Not what I was expecting at 23," Ms Danher recalled.

"We were with him more often than our other children. He loved his mummy and daddy."

It had been common for Liam to suffer seizures every few weeks and the couple had applied to the NDIS for a specialised seizure mat that would have sounded an alarm in the case of an epileptic fit.

A portion of the letter Liam Danher's parents have written to NDIS Minister Linda Reynolds.

But the Danhers said their application was met with knockbacks and requests for more documentation to justify the $2,500 mat.

The Danhers claim that lawyers acting for the government were trying to find reasons to have the application rejected.

"Where's the human side? Where's the empathy?" Ms Danher questioned.

"It just makes me want to cry," Mr Danher added.

NDIS Minister Linda Reynolds said she was saddened by Liam's death. (ABC News: Matt Roberts)

Also in their letter to Senator Reynolds, they demand an apology over Liam's death and an independent investigation.

Senator Reynolds told Senate Estimates she would be happy to meet with Liam's parents.

"And I will be very happy to meet with [Liam's parents]."

The Danhers said they would meet with Senator Reynolds when they received her offer.

National Disability Insurance Agency (NDIA) CEO Martin Hoffman told Senate Estimates that Liam's death was a "complicated and terrible situation".

When questioned by Labor senator Nita Green, Mr Hoffman refused to provide any detail on Liam's case, but said there was an internal review underway.

"We're considering the information we've gathered in terms of the interactions between the [NDIA], the family and service providers," Mr Hoffman responded.