Rotorua woman Rachel Saunders was travelling on the bus that the boy joined and said he was remarkably calm for having been left behind.
"He looked a bit rattled but he's quite a confident little kid ... I don't know how long he'd been at the gas station, but he said that he approached a lady who had a lot of make-up on because she seemed to look quite safe to talk to."
Saunders said they asked him what happened and he said he told the bus driver he really needed to go the toilet.
"We were just chatting away and said what happened and said 'I told the bus driver I needed to go to the toilet and he just took off, he left me, he left me'. He wasn't crying or anything like that."
The boy, who said he was from Papamoa, said he knew the bus wasn't stopping long but when he got to the toilet there was a queue of about seven people.
"So it took a bit longer and when he came out the bus was gone.
"He was like 'oh karma is going to get that driver'. He didn't look like he had been crying or anything like that I think he was just in shock."
He was worried about his bags, one which had his snack supplies inside. However, a teenage girl he was sitting next to shared her crackers and he seemed "quite chatty".
Saunders said she got off at Auckland Airport but the teenage girl said she'd look after the boy until his final stop at Sky City.
"He was in good hands ... he was just gobsmacked that the bus driver wouldn't wait the extra two or three minutes that it took him to go to the toilet."
She said she appreciated buses had to operate on time but she couldn't understand why.
"'The fact that he actually spoke to the bus driver is what everyone is so annoyed about. it wouldn't have taken much for him to just wait for the little boy to get back on. He was quite confident but he just looked relieved when he sat down."
General manager for New Zealand Coachlines and Auckland Tourism Sam Peate said InterCity acknowledged the incident took place.
Peate said an investigation into the incident was under way and they had spoken with the family of the child. His ticket had also been refunded.
Meanwhile, other InterCity passengers have also shared their horror stories with the Herald.
Tauranga woman Chanchal Saraswat said her 7-year-old son has been left traumatised after a bus driver said he wouldn't wait for them if they went to the toilet.
She said the pair, along with her husband and 2-year-old, drove up to Auckland last month, however her son wanted to bus home. She got tickets and they boarded the bus, however when it stopped at Bombay her son desperately needed to use the loo.
She asked the bus driver who said they would only be stopping for two minutes. They ran to the toilet but then she noticed the driver closing up the bus so they went back, without using the toilet.
Her son managed to "mostly" hold on but he sat in considerable pain until they got to Thames where a group of passengers used the facilities.
The bus arrived in Tauranga 10 minutes early, she said.
However, she never made an official complaint which she now felt guilty about it, especially after reading about the 12-year-old boy's plight.
"I feel so sorry for the boy and feel guilty in a way by not complaining then. I request other passengers to please share if they've had a similar experience."
Hamilton man Malik Awan was also ditched in freezing temperatures in Bulls after using the toilet. He said the driver told him the bus would leave at 5.05am however when he returned at 4.53am the bus had already gone.
The 42-year-old IT specialist said he missed a vital job interview. He was yet to complain but after catching a ride with a member of the public was eventually put on a bus in Levin for the end of his trip to Wellington from the Waikato.
