Talking on 3AW radio on Tuesday, Lou said adrenaline kicked in when he saw a car plough into pedestrians.
Lou suspects having trained at the Australian Defence Force Academy for a short time helped him keep a level head and direct those nearby.
"It was basically, right, this is what needs to be done, bang, bang, bang, you there, you there," he said.
He began treating four people around him when his attempts to flag down police went unnoticed.
He saw an off-duty orthopedic surgeon using brochures to strap a woman's leg, while workers from a nearby pharmacy rushed over medical equipment. He ran up and down the length of the footpath seven or eight times, checking every single individual, he said.
Lou even made an effort to get water bottles for the police officers as they worked.
Nineteen people were still in hospital on Tuesday after a man allegedly deliberately drove into the lunchtime crowd, killing five people and injuring more than 30.
James 'Jimmy' Gargasoulas, 26, was charged with five counts of murder on Monday, and is likely to face more charges.
- AAP