Safety measures at the Burt Munro Challenge will need to be looked at after five people were injured when motorcycles crashed into crowds, an organiser said.

One person was critically injured, three seriously and a fifth was taken to hospital in a moderate condition after two separate crashes at the event's Bluff Hill Climb, Southland, on Thursday.

Several others were treated at the scene.

"One of the bikes had gone round the second corner and he had come off and the way he's come off, it slipped, and he's gone into the crowd," organising committee chair Craig Hyde said. The rider was not injured.

There are barriers on the hill, and crowd marshalls, but safety measures would need to be reassessed in light of the crashes, he said.

Spectators may be prevented from going up the hill to watch in the future.

"It's something we're really going to have to evaluate, whether we actually have people up the hill again to watch next time."

Racing had resumed the first crash, in the morning, after riders were spoken to, police attended and measures were evaluated. Following the second crash was in the afternoon all spectators were removed from the hill, Hyde said.