Warm weather, rivers of melting tar

A109 Light Utility Helicopter flight with mayor Gisborne City from the air in November 2023.

Can someone from NZTA or the roading industry explain why it is that as soon as there is more than one day of warm — not hot, but warm — weather our roads turn into rivers of melting tar?
State Highway 2 at Morere is a prime example where the road has become a river of bleeding tar, causing huge issues for trucks; resulting in the road being blocked at times as trucks become stranded. 
In the past we have heard that this is a result of the solvents used in the tar evaporating off. Is it a case of the tar being “watered down” too much, or a sub-standard tar being used? 
You would think that the tar being used would be appropriate to the traffic and climatic conditions of the road, but perhaps that is too sensible . . . 
Can’t wait to see the tar-splattered vehicles rolling in to Gisborne for RnV, sure it will be a good advertisement for the area!

Michael Arnaboldi

Footnote from Ed: We put this letter to NZTA/Waka Kotahi along with similar concerns being raised by others. A response from them is in a story on page 3 of today’s paper.