A 21-year-old man has been flown to hospital after suffering an axe blow to the head in a mock medieval battle at the Hawkesbury Showground in New South Wales.

Mitch Toohey, from Ballarat, Victoria, suffered concussion and a fractured skull when an axe hit his helmet during an organised battle at the Winterfest Sydney Medieval Fair.

Mitch Toohey was injured in a mock battle. ( Facebook )

A CareFlight rescue helicopter was dispatched to the scene at about 11:00am.

Mr Toohey was flown to Westmead Hospital in a stable condition.

A witness at the event, Wayne Briggs, said Mr Toohey was fighting with another man when a third person ran across the field and struck him in the head with an axe.

"His head bounced off his shoulder and knocked him out," he said.

Mr Briggs said Mr Toohey continued to be struck until a referee intervened and he was taken off the field.

The president of the Australian Medieval Combat Federation and event marshal Stephen Slater said incidents do occur in medieval mock battles.

"Like any full contact such as MMA or football, injuries can happen.

"Obviously there's a lot of regulations put in place to minimise it, from weapons standards, armour standards, to also marshals like myself who are the referee."

The Winterfest event includes mock medieval battles. ( Supplied: CareFlight )

Winterfest event co-ordinator Andrew Beattie said the CareFlight helicopter was called as standard practice for dealing with a head injury.

He said incidents like this were not something they wanted to happen but contact sports carried risks similar to football.

"There is an inherent risk, but everyone's having a great day."

Winterfest, which concludes on Sunday, is billed as a festival that "celebrates medieval and fantasy culture".

The festival's website says re-enactment is a "core part" of Winterfest.

"It is an educational and immersive activity where the people involved enjoy recreating aspects of historical events or periods."