Masterton police have made a public plea for anyone who saw or knows anything about the incident to come forward, including the driver of the car.
Senior Sergeant Warwick Burr said Anstis was heading south when a car, also travelling south, hit him.
It was possible the driver was unaware his vehicle had spilled the cyclist on to the road because it was dark and the bicycle had no rear light.
Anstis was admitted to Wairarapa Hospital and yesterday underwent surgery on his leg which included a pin being inserted.
Less than two weeks ago Anstis was in the sporting spotlight when he won his AWD events over 200m and 400m at the Wellington track and field championships at Newtown Park, Wellington.
He went on to show his versatility by also winning the AWD long jump.