Gritting lorries have been out on the Island's roads this morning (Tuesday).

Primary routes were due to be pre-salted from around 5am.

The operation was left until the early hours this time, as temperatures were due to fall overnight and treated roads were thought likely to freeze over.

More than 5,000 tonnes of road salt has been amassed at the Department of Infrastructure's Ellerslie depot, and crews have been on stand-by to cope with severe weather since last month.

Alan Hardinge is deputy civil engineering manager, overseeing the operation to keep the Island's traffic on the move.

He says there are different factors to consider when choosing the best time to send crews out: