The inquest into her death was into its seventh day on Wednesday. The central question comes down to whether the compressed air which powers the brakes ran out and if so, why.
Experienced mechanic David Taylor looked at the bus after the crash. He was the second mechanic who found no mechanical issues.
"When we subjected the bus to the brake test at VTNZ, the brakes worked perfectly [satisfactorily]. All the steering joints were connected; the tyres, the suspension were all in good order," Taylor said.
Other than Hannah, 18 of the 31 people on board were injured, three seriously.
No-one was ever charged over the crash. The driver gave evidence earlier but cannot be named.
On Wednesday, NZ Transport Agency heavy vehicle engineer Alexander Currie outlined how using the foot brake too much could have drained the air from the system.
He was asked questions by lawyer Matthew Ferrier, acting for the bus operator Ruapehu Alpine Lifts:
Ferrier: "So too high of a gear means the engine's not revving to drive the compressor?"
Currie: "Correct."
Ferrier: "And the repeated or sustained application of the service brake using up the air in the system?"
Currie:> "Correct."
Ferrier: "And because of the high gear, the compressor's not able to compensate?"
Currie: "Correct."
Currie said there was clear photographic evidence that showed the foot brakes were used heavily, instead of engine braking.
The bus, a Mitsubishi Fuso, was an appropriate vehicle and had not been involved in many crashes, he said.
"They're not over-represented," Currie said.
Ferrier asked: "In your view designed to operate in all conditions, including on the Ōhakune Mountain Rd?"
"Yes," Currie replied.
A different view
However, a mechanic brought by the driver's lawyer said it was highly possible the driver accidentally bumped a switch - which was not installed by the manufacturer - which subsequently allowed all the compressed air in the brake tanks to be released.
The mechanic, Mike Brown, said the driver was experienced, with approximately 500 trips up and down Mt Ruapehu under his belt.
Something out of the ordinary happened on the fatal trip, he said.
An inadvertent knock of the auxiliary switch, which would have been easy to do, would have left the bus without brake power within 30 seconds, he said.
The switch sat next to the driver's armrest and was a left over from an old system which was not meant to be in use any more.
The auxiliary switch had played a "critical role" in the loss of braking power, he said.
Brown also said the bus was not fit for purpose, was too old, was not designed for alpine environments, and had a number of worn parts.
While questions about the cause of the crash - and how future crashes could be avoided - are yet to be determined, how to improve safety during a crash also came up.
Counsel assisting the coroner David Boldt raised a common question: Whether seatbelts should be compulsory on alpine buses.
He posed that to Bus and Coach Association chief executive Dr Pim Borren.
"My view would be that it would be vastly preferable for alpine operators to have seatbelts and for passengers to be wearing them," Borren said.
Borren said there was some industry resistance to that idea because most buses did not have seatbelts, and it would be "economically challenging" if changes came in quickly.
The inquest in Auckland before Coroner Brigitte Windley continues.
