The acrid smell of burning rubber and a couple of strange noises were the first signs that Faridah Merchant's morning bus ride was about to go terribly wrong.
The 22-year-old was sitting near the back of the L94 bus, making a regular trip from the eastern suburbs to her work at a women's accessories store in the city, when a woman yelled: "Oh my God, the bus is on fire".
Ms Merchant saw plumes of black smoke out the window and said the bus "got very, very hot", before the driver pulled over on Oxford Street, Darlinghurst, about 9.30am today.
Pedestrians and early morning patrons in the Courthouse Hotel saw the bus rapidly pull up to the curb outside the pub, with smoke and flames streaming from the back window.
"We could sort of hear the engine making a horrible sound and then the bus driver pulled over to the side of Oxford Street," Ms Merchant said.
"Everyone was really confused at first, no one knew what was happening. Everyone was just panicking a bit.
"After that everyone just got up and started moving towards the door.
"The bus driver was panicked as well, his face went red, and he didn't know what to do.
"It was just horrible.
"I've never been in such a situation in my life. You don't think it's ever going to happen to you.
"I was very shaken. I'm OK now, but I'm still in shock.
"I'm kind of put off ever going on a bus after this."
The bus driver was panicked as well, his face went red, and he didn't know what to do. It was just horrible. I've never been in such a situation in my life. You don't think it's ever going to happen to you. I was very shaken. I'm OK now, but I'm still in shock
Another passenger Greg Pallier, who was sitting on the backseat, said he noticed smoke rising from the floor of the bus and yelled out to the driver: "Hey mate, your bus in on fire".
"I was just playing with my iPhone and noticing the smell and then I started seeing some smoke coming up through the trapdoor ... on the floor.
"That's when we yelled out to the bus driver."
Mr Pallier said most people appeared calm as they piled out of the back and front doors to catch another bus into the city.
He called triple-0 and watched as flames eventually rose up over the roof of the bus.
"When we got off the bus you could see flames dripping onto the ground.
"It was a pretty hot fire. It got really hot really quickly."
None of the 30 passengers were injured, while about 50 people were evacuated from nearby shops.
Authorities spent an hour examining the bus, which was left with a shattered back window, melted indicator lights, a badly bent and charred engine door and peeling plastic signage.
Police said the cause of the fire was still under investigation.
Lucius Tweedie, 23, of Potts Point, said when the bus pulled up its passengers "scattered like ants".
"The whole back of the bus was burning, on fire.
"It's gone inside the bus. The actual fire was below the window at the back.
Mr Tweedie was walking on the other side of the road when he saw the fire.
"From what I gather when the bus got here, there was an explosion at the back.
"It looked like [the back of the bus] was bowed out.
"It's the scariest thing."
A bartender at the Courthouse Hotel, who asked not to be named, said the bus came "smoking around the corner".
"There were a couple of little pops," he said.
"There weren't massive flames.
A patron said: "Just another day at the Courthouse".
Last October, an explosion on a gas-powered State Transit bus triggered snap industrial action, when the union representing bus drivers instructed them to take them out of service.
But the bus that caught fire this morning is understood to be powered by diesel, not gas.
"If it was a gas bus we would be lodging a complaint in the commission [Industrial Relations Commission]," said the RTBU's bus division secretary, Chris Preston.
Mr Preston said the bus that caught fire this morning was a Scania diesel-powered bus.
"I would say that there's no history with that particular model bus catching on fire," Mr Preston said.
"I think it might be just one of those things."
- with Jacob Saulwick