The Mercury understands a 28 year old motorbike rider and a 14-year-old mountain bike rider injured themselves at roughly the same time, but in separate incidents, on the mountain bike trail known as Possum's.

NSW Ambulance paramedics were called about 3.20pm Monday.

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The teen was knocked unconscious after colliding with two saplings. His helmet was badly damaged on impact but he is now conscious and communicating with emergency services. His friend, who had been riding up ahead, raised the alarm but was unable to pinpoint his exact location for first responders.

Paramedics found the teen about 800 metres into the bush.

It took responders some time to confirm that emergency calls coming from the site related to two separate incidents.

The 28-year-old rider was located about 200 metres further into the bush, and is also now conscious.

A NSW Ambulance rescue chopper is surveying the area and may play a role in the retrieval, with Doonan Park cleared as a likely landing zone.

Eight paramedics, four firefighters and one police officer are in the bush.

Update, 4.25pm: With night falling, the helicopter crew is preparing to winch in a specialist paramedic to one of the injured riders.

The Mercury understands the other has been loaded onto a Stokes Litter and will be carried out by firefighters and paramedics.

The incident has drawn dozens of residents to Doonan Park.

The injured teen is to be airlifted to Westmead Children's Hospital.

More to come.

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