Giving evidence in his defence on Wednesday during a Newcastle District Court sentence hearing, Button, 59, admitted taking more than the prescribed amount of the opioid Tramadol before the crash.
"I'm ashamed," Button told the court.
"I can't forgive myself. I can't believe I caused this. I never meant to cause this.
"I truly wish it never happened."
The crash killed 10 and injured another 25 people, all guests at a young couple's wedding. (Darren Pateman/AAP PHOTOS)
Button said he had tried to figure out the words to say he was sorry but how could he say he was sorry for causing such a "horrible and tragic event" which had ruined the lives of hundreds of people.
He said he had committed the "ultimate sin" and caused parents to have to bury their children.
The biggest nightmare of his life was to have to bury his own children.
"I hate myself. I want to disappear," Button said.
"I'm sorry to everyone involved. I've shamed everyone."
Judge Roy Ellis said Button had an obligation to protect and maintain the safety of the 35 people on board the bus.
The judge said he considered Button had abandoned his moral responsibility to a significant degree.
Button claimed he had been using Tramadol since 1994 and had developed a tolerance to the opioid he used to relieve pain.
"I wouldn't have taken it if I thought it was a risk to my driving ability," Button said.
He had taken a total of 350mg of Tramadol on the day when doctors told him the daily maximum dose was 200mg.
Questioned by prosecutor Katharine Jeffreys, Button agreed he had been joking about going on a rollercoaster before the crash and knew he was a danger on the roads if he had taken more than the prescribed amount of Tramadol.
Button has pleaded guilty to 10 charges of dangerous driving causing death, nine counts of driving causing grievous bodily harm and 16 counts of causing bodily harm by wanton driving.
During the three-day sentence hearing, Jacqui Varasdi, mother of one of the 10 fatalities, Zachary Bray, 29, questioned why the manslaughter charges against Button had been dropped by prosecutors.
A victim's mother has questioned why manslaughter charges were dropped against Brett Button. (Mark Russell/AAP PHOTOS)
"I put my trust in the judicial system and the professionals appointed to represent us," she said.
"However, my family and I feel left down by those representing us, specifically the DPP following the decision to drop the 10 manslaughter charges without any adequate consultation and opting for a lesser plea," she said.
"This decision has been even harder to accept as it was after the plea deal that we learned the driver was impaired with high levels of opioids and painkillers in his system.
"His actions were just not reckless.
"They were criminal and warranted a manslaughter conviction.
"He knowingly over-medicated and chose to drive under those conditions putting countless lives at risk and destroying so many."
A number of other families have also criticised prosecutors for making the plea deal with Button.
Manslaughter charges carry a 25-year maximum jail sentence while the lesser charge of dangerous driving causing death carries a maximum 10-year jail term.
Button lost control of the bus taking 35 wedding guests from the Wandin Valley Estate to Singleton about 11.30pm on June 11 last year after taking a roundabout on Wine Country Drive at Greta too fast in thick fog
He accepted his driving ability was impaired by the opioid, which can cause drowsiness, mental clouding and visual functioning deficiencies.
The 10 people who died in the crash were: Darcy Bulman, Nadene McBride and her daughter, Kyah, Kane Symons, Andrew Scott and his wife Lynan, Zach Bray, Angus Craig, Tori Cowburn, and Rebecca Mullen.
Many on board were members of the Singleton Roosters Australian Rules Football club and had earlier attended the wedding of Mitchell Gaffney and Madeleine Edsell.
The sentence hearing continues.
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