Labor’s promise to reintroduce a scheme for motorists to get their tolls back from using Sydney’s M4 motorway would deliver a “free kick” to the new WestConnex owners' earnings.

Analysts and academics also expect a surge in traffic from a “cashback” scheme on the widened M4 between Parramatta and Homebush to funnel more motorists onto the rest of the WestConnex toll road in which private investors will soon gain a controlling stake.

Premier Gladys Berejiklian has described Labor's plans to reintroduce a cashback on the M4 as a 'con'. Credit:Louise Kennerley

And they say the prospect of a toll subsidy on the M4 if Labor wins the state election next March raises questions about inequity if cashbacks do not apply to other toll roads such as the M7.

New figures show the annual bill for taxpayers of refunding motorists who drive on the M5 South West – the only toll road in Sydney to which a cashback scheme applies at present – rose to $111 million in the 12 months to June, from $105 million previously.