The woman had a screen and a keypad on her lap and looked like she was strapped in.
"I didn't know if I had time to undo clips or anything ... It was the whole chair or nothing ... literally the train was coming round the corner," the 24-year-old told the Herald.
The wheelchair was jammed in the tracks, so all he and the jogger could do was push it over.
All three collapsed backwards as the train roared by. The woman was dragged by her wheelchair - which had been hit by the train - along the concrete, while the one of the women rescuers was left bleeding from a leg injury as she fell clear.
Mr Mereau felt the rush of the train and braced himself.
"Thinking about it since then I was so lucky not to be pulled in by something else sticking out of the train. It was crazy."
Staff and patrons of a nearby gym also rushed to help in the aftermath.
Afterwards, the train driver told Mr Mereau he thought he'd "wiped all three of us out".
"It would have been awful for him, he would have only had about four seconds and there was nothing he could do."
Mr Mereau believed if he was only a minute later the woman would have surely died, which was a frightening thought, as was the realisation of how close he came to death.
"I was actually really lucky not to be hurt. It was such a close shave."
KiwiRail said the crossing was upgraded in 2011 but CCS Disability Action adviser Vivian Naylor said she would be very reluctant to negotiate it in her conventional wheelchair.
Albert-Eden Local Board member Graeme Easte said there was an urgent need for Auckland Transport to allocate a budget for the progressive "grade separation" of crossings throughout the Super City.
The organisation is understood to have been working for several months on a report ranking crossings in most serious need of separating roads from the railway.
Police and KiwiRail have launched an investigation into the incident.
Read more: Trio's brush with death on rail line