A pall has been cast over a small country town after 13 people were injured when a demolition derby car ploughed into spectators in a "freak accident".

Emergency services descended on Walcha's showgrounds, 90km from Tamworth in NSW, after a crash at the weekend Walcha Motorcycle Rally.

An on-track collision in the demolition derby sent a speedway car with a 27-year-old driver crashing through the fence and into a mobile grandstand, NSW police said.

Thirteen people were injured in the crash.

Two were flown to John Hunter Hospital in a critical condition - a man in his 50s suffered spinal and hip injuries and a man in his 30s had a head injury.

A hospital spokesman told AAP on Sunday morning the condition of the men had improved to serious but stable.

The 11 other victims, aged 20 to 7

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