Mystery surrounds critical injuries suffered by a Sunshine Coast cyclist, with police unable to determine if he was knocked from his bike by a vehicle.

The 50-year-old man was found unconscious on the roadside by a passerby in rural Kiamba, north of Nambour, about 5.15pm on Friday.

Police are calling for help after a cyclist was found unconsicious on the roadside at Kiamba.

He was flown to the Royal Brisbane Hospital with life threatening head and chest injuries.

Police have appealed for information as they work to determine what happened to the man.

Anyone with information is asked to phone CrimeStoppers on 1800 333 000 or go to crimestoppers.com.au

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