A cyclist has described the "incredibly frightening" moment he was hit by a truck in Sydney's south at the weekend, knocking him to the ground and leaving him with serious injuries.

Michael Long, from Annandale, set off for a regular 6am ride from Waverley to Sutherland on Saturday morning with around 24 others from the Eastern Suburbs Cycling Club.

The group had been on the road for about 15 minutes and were cycling southbound on Southern Cross Drive in Eastlakes when a truck ran into the back of them, leaving two of the riders in hospital with serious injuries.

Mr Long, 55, told Fairfax Media he was riding second to last in the group, which was travelling two abreast. He was chatting with the rider next to him when suddenly he heard the beep of a horn and a sickening crunch.