A North Canterbury arsonist has escaped a fine for setting a pile of 400 thousand tyres alight in 2018

Garry Grimmer has pleaded guilty in Christchurch District Court to a charge of arson for the Amberly blaze that took two days to subdue.

The 62-year-old says his frustration boiled over after two years of watching the tyre pile grow.

Grimmer says he thought the old tyres could be contaminating the town's water - and snapped when the authorities continued to do nothing.

He's avoided having to pay costs - but Judge Jane Farish has sentenced him to nine months' detention.

She didn't believe Grimmer's actions were premeditated.