The cost to the NSW government of paying a private consortium to operate Sydney's new light-rail line for 15 years has surged by 70 per cent, blowing out to almost $938 million.

In a further blow to the project, Transport for NSW has blamed the significant increase in the operating cost from its estimates before it was put to tender "in part on the more superior level of service than [was] originally anticipated".

The operating cost is separate from the $2.1 billion capital cost of building the 12-kilometre light-rail line from Circular Quay to Kensington and Randwick in the city's south-east and buying the trams that will run on it.

The latter blew out by $549 million in 2014 due to "mispricing and omissions" in the business case a year earlier – not "customer improvements in the original scope" as the government had claimed at the time.