Trucks will face stiff fines for using Pennant Hills Road in northern Sydney instead of the NorthConnex toll-road tunnel under construction between the M1 and M2 motorways but the state government is yet to reveal exactly what the penalties will be.

Declaring construction of the 9-kilometre twin tunnels at the halfway point, the government talked up the benefits of the $3 billion project taking about 5000 trucks a day off the heavily congested Pennant Hills Road when NorthConnex opens to traffic in late 2019.

Cars will pay tolls of $7.02 in today's dollars for driving on NorthConnex, while trucks will be charged three times more at $21.05. Like those for the M2, the tolls will rise at 4 per cent a year or the rate of inflation, whichever is greater.

NorthConnex will be the only project in NSW that imposes fines on truck drivers if they use a free alternative such as Pennant Hills Road.