Illawarra Bicycle Users Group chairman Werner Steyer said motorists had complained to NSW Roads and Maritime Services about cyclists riding in packs.

"This is rather than riding single file and leaving room, wherever possible, to allow people to pass," he said.

"It's mainly impatient drivers because the opportunities for overtaking riders on Macquarie Pass, the same as a lot of roads, are somewhat limited.

"So if [cyclists] get themselves in a position where they ride in a pack, it frustrates drivers to the extent that they then take unnecessary risks, which puts everybody at risk, like overtaking on the wrong side of the road across unbroken centre lines."

Mr Steyer said the problem could be compounded by motorists underestimating cyclists' speed while overtaking.

"You get people trying to overtake you doing 45-kilometres/hour in a 50-kilometre/hour zone and not realising by the time they get to where they want to turn off, they've got the brakes on," he said. "Most road cyclists will be doing more than 30-kilometres/hour on the flat and it takes a long time to get past in a car."

South Coast Highway Patrol Sergeant Nicholas Park said police dealt with motorcycle accidents more often than bicycle collisions, but warned cyclists riding downhill could build up as much speed as a car.

"People need to be careful from that point of view," he said.

By law, cyclists can ride no more than two abreast, but Mr Steyer said single file was often a better option.

"If you're in an area where there's multiple lanes, like if you're riding along Shellharbour Road, there's no reason why you can't ride two abreast," Mr Steyer said.

"But in other places where people want to ride two and three abreast and just choke up the road, it's inappropriate."

He said riders on Macquarie Pass should stay as far left as possible to maximise overtaking opportunities for motorists and have lights on their bicycles to ensure visibility in low-light areas. "I think the issue is not whether you ride, drive or what, it's more a matter of being courteous and considerate of other road users," he said.

An RMS spokeswoman urged all road users to share the road.

"Motorists should always overtake safely," she said.