Tar is melting on the roads in some places, yet the New Zealand Transport Agency is warning drivers to be wary of ice on the road.

Summer ice, that is.

While motorists in colder climates were used to warnings of black ice during the winter months, the agency warns that it is peak season for the road condition they call "summer ice" - where dust, dirt, and oil build up on the road.

The agency says when it rains, the road became greasy, making it slippery, and the greatest risk was shortly after rain starts.

Agency spokesman Andy Knackstedt said he was not sure how widely the term "summer ice" was used overseas.

"The phenomenon which it describes, slippery roads from accumulated dirt and oil after summer rain, is certainly something which is an issue overseas as well as in New Zealand.

"The term has been used by police and NZTA in New Zealand for at least a couple of years."

Knackstedt said the agency did not have specific data on how many crashes had been caused by summer ice.

"We do know that at this time of the year, when we have long dry spells, dust, dirt, oil and other materials can build up on the road surface, and when it rains, the road surface can become greasy, making it very slippery.

"We encourage drivers to be aware of this risk."

The agency says there was no way to remove summer ice from roads because it builds up over time, but it would issue warnings about it.

It says motorists can be "summer ice smart" by keeping speeds down on wet roads after a long dry period, taking extra care on curves, increasing following distance from vehicles in front, braking and steering gently, and allowing extra travel time in case they encounter summer ice conditions.

-Stuff/Rachael Kelly.