Firefighters watch as blaze tears through tyre yard in North Canterbury

Firefighters are watching over a blaze in North Canterbury as it tears through around 20,000 tyres.

Crews were called to the scene on Racecourse Road in Amberley just after 6pm on Monday night to find the tyres well ablaze.

Canterbury assistant area commander Mike Murray says dousing the flames was not an option, as the runoff could potentially enter the area's water supply.

Mr Murray says crews are instead monitoring the fire and letting it burn itself out.

"It's the lesser of the two evils."

Residents in the area have been warned to close their doors and windows due to smoke in the air, but there is not considered to be any threat to people or property.

The Hurunui District Council advises those who have symptoms of concern or breathing difficulties to seek medical advice.

It is not known how the fire started.