The national day of mourning for Queen Elizabeth II has brought the school holiday crackdown forward, with police enforcing the penalty on speeding, seatbelt, mobile phone and motorcycle helmet offences from midnight on Tuesday.

Transport for NSW Deputy Secretary for Safety, Environment and Regulation Tara McCarthy reminded drivers to stay cautious on the roads ahead of the busy school holiday period.

"Although this is a sombre occasion, there is still likely to be more people out on the roads over the next few days and over the weekend with the school term ending on Friday, so it is important we all take extra care," Ms McCarthy said.

"The last thing we want to see over this period is an increase in road trauma, which is why double-demerits will be in place like they would for any other public holiday to deter people from doing the wrong thing and putting lives at risk.

"Drivers are reminded that this period includes some school days and it is especially important to slow down and take care in school zones at school times - even if you cannot see children near the road."

Double demerits will apply for all speeding, seatbelt, mobile phone and motorcycle helmet offences from 12am on Wednesday, September 21 to 11.59pm on Sunday, September 25.

The school term continues on both Wednesday and Friday, meaning school zone speed limits will be in force.

Legislation means double demerits automatically apply on a long weekend when a public holiday falls on a Thursday, Friday, Monday or Tuesday.