The private operator of Sydney's NorthConnex motorway below Pennant Hills Road will receive compensation from the government if too few trucks use the tunnel.

But the government will not say how many trucks are required to use the NorthConnex tunnel before compensation will be paid, nor how much toll-road company Transurban might receive in compensation.

The operator of NorthConnex will be paid compensation if not enough trucks use the 9 kilometre motorway

The state government has said that when NorthConnex opens in 2019, it will fine trucks that avoid the toll-road by running on the surface roads but do not have a reason to do so.

The fines will help ensure trucks do not use local roads, but will also ensure an income for Transurban, which is contributing the lion's share of funds required to build the $3 billion road.