The latest cost blowout for the project, from $15.4 billion to $16.8 billion, seems attributable to add-ons and elements of the motorway that have turned out harder to build than planned. The cost of WestConnex has ballooned since a $10 billion project was proposed in 2012. The cost now sits closer to $17 billion. For instance, the business case says an extra $1.4 billion will be spent on connections to the Anzac Bridge and Victoria Road, as well as connections to a possible new harbour road tunnel between Rozelle and the north shore. About $300 million of the blowout is attributed to "acceleration costs", while about $400 million is put down to extra costs for the "Sydney Gateway" – the difficult road connection between the motorway and Sydney Airport and Port Botany, which was the original justification for the greater WestConnex project. The business case shows the Sydney Motorway Corporation, the entity in charge of WestConnex, does not yet have a completed design for this part of the motorway.

"It is dependent on further development work, so timeframes may change," the business case says of the road connection between Sydney Airport and the motorway. "At the latest, it will open by 2023." One of the striking aspects about the revised business case is the huge reduction in traffic predicted for the current M5 East motorway. That is largely because a toll will be introduced onto the motorway for the first time. The business case predicts a reduction of 60,000 vehicles per day using the existing M5 East by 2031. More than half of them – about 37,000 – will use the duplicated M5 East, which is part of the WestConnex motorway project. But thousands will avoid the toll roads completely. The business case also predicts traffic will increase on the surface roads such as along Victoria Road at Rydalmere and Parramatta Road at Auburn because motorists will want to avoid the tolls on WestConnex. NSW Roads Minister Duncan Gay said the revised business case showed a strong economic case for the project, and daily travel time savings for thousands of motorists and road freight operators.

"The updated business case describes enhancements to the original project design, including aligning WestConnex to a future second road tunnel under Sydney Harbour," he said. According to the business case, WestConnex will encourage an increase in driving through Sydney. By 2031, vehicles will travel an extra 600,000 kilometres in Sydney as a result of the motorway. But the business case says the total hours driven will fall by about 110,000 a day - suggesting trips will be quicker. And the document also highlights significant difficulties in trying to integrate the motorway local roads. At Rozelle, Concord, St Peters and Haberfield surface road congestion "may impact on the performance of the interchange," the document says. The entire WestConnex motorway is due to open in 2023. Construction has already started on the first element - a widened M4 motorway between Parramatta and Concord. "WestConnex is the largest urban motorway project being built in the southern hemisphere, creating 10,000 jobs – including trade apprenticeships – many based in Western Sydney," the Federal Minister for Major Projects, Paul Fletcher, said.

While still supportive of the project, the NRMA said it was concerned that tolls would rise by 4 per cent for the first two decades of operation, which was potentially more than the consumer price index. NRMA president Kyle Loades reiterated the motoring group's call for tolls to be set by an independent body and linked to CPI. "The motorist will bear the cost of WestConnex. If motorists believe that the toll is set too high, Sydney's experience is that people will simply stay away as is the case with the Cross City Tunnel," he said. WestConnex Action Group spokeswoman Pauline Lockie said the cost of the project was blowing out to the tune of $1.5 billion a year before even a sod had been turned. "How far down the path are we going to have to go before we realise this is a dud for the NSW taxpayer?" she asked.

Ms Lockie said the escalating cost of the project raised serious questions about how the outlay would ever be recovered over the coming decades. "There are huge costs in tolls and in the huge social and health costs that NSW will have to live with if this is built."