Two trucks have collided in Greta Valley, north of Christchurch.
The incident happened at around 9am on Greta Rd, in the Hurunui region.
Diversions have been put in place and Fire and Emergency NZ were on the scene.
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Sgt Rene Pabst said a gas truck was struck from behind, pushing it off the road. He said 35 gas canisters came off the back of it.
"There was a danger of them being able to explode but the fire service advised us that they are safe, but there's still gas escaping from them."
The Serious Crash Unit is attending the scene.