Police are urging drivers not to be complacent about the dangers of black ice after two recent crashes.

"Obviously the roads in the last couple of days have been pretty atrocious," Police Southern Communications shift commander Kelly Larsen said.

Emergency services today attended two crashes near Summit Rd south of Christchurch.

Ms Larsen said even drivers familiar with icy roads should take extra care.

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"The hazard is black ice...chains aren't probably going to help significantly."

She said keeping speed down was one way to minimise risk.

One car went off the road near Tai Tapu and black ice was suspected as contributing to the crash.