Acrow, which is contracted by V8 Supercars Australia to dismantle the track and which employed the man, said all work on the V8 site would stop until the investigation was completed.
Police were at the site last night, and it was also being patrolled by security guards.
Acrow managing director Mark Irvine said other employees were extremely shaken.
A Willoughby St resident, who did not want to be named, said she heard an "almighty crash and thought half the stand had collapsed".
From her driveway she could see a man lying on the ground.
He had been moved several metres away from the grandstand he had fallen from.
She said men who had been installing cabling on her street rushed to the entrance of the fenced-in stadium to help.
Mill St resident Odette Haitana, who can see the grandstand from her lounge, described a long loud bang sounding like "steel on steel" when the accident happened.
St John Hamilton team manager Cath Shannon said the man was unconscious when paramedics arrived and was taken to Waikato Hospital in a serious condition.
