"The response was absolutely shocking, it was just terrible. It was easily 45 or 50 minutes before he got any attention," he said.
He said the new skate and BMX park, which opened in November in Waterview near the North Western Motorway, should not have taken an ambulance long to get to.
"It was very clear they had no idea where they were. We didn't want to keep on their case, but at regular intervals we called back saying 'where are these guys?'
"[The injured man] started flicking into shock quite badly - hyperventilating, convulsing, his eyes were rolling back. The pain was becoming too much. And he's a big guy, a very strong guy.
"It was terrible to watch, it was really, really hard."
St John spokeswoman Jennifer Porter said the first emergency call was received at 6.26pm. She said the patient was assigned as low acuity, meaning non-life threatening injuries.
After a conversation with the patient at 7pm, the status was upgraded, and an ambulance arrived on scene at 7.16pm.
"The response was within our guidelines of acceptable times," Porter said.
Jacob said the pump section of the park, a set of short rolling bumps used by BMX riders, was not designed correctly and was causing injuries - including the man's broken leg.
"It really is dangerous. It's almost like the whole [pump section] has been compressed. The gradient's really steep. It's way too tight," he said.
"Quite a few of the parents don't allow their children there."
This meant it was easy for riders to hit their chest into the handlebars, or even fly over the top of them, he said.
Another witness Lisann Harris was at the park with her two children when the accident occurred.
She said she had spoken to older, proficient BMX riders about whether the pump section was safe for her kids to ride on.
"We spoke about it even before the accident. They said, 'Man, that is so tight'," Harris said.
NZTA, which was responsible for building the park as part of the Waterview motorway project, said: "The skatepark which is designed for intermediate and advanced skateboarders was built by ConVic an Australian firm with a global reputation for developing recreational facilities for youth."
Auckland Council spokeswoman Agnes McCormack said: "The council has not received any calls regarding complaints or concerns about the safety of the skateboarding facilities at Waterview BMX and skate park."