With a population boom under way in suburbs along the banks of the Parramatta River, the state government is buying four new ferries to meet growing demand for services from residents wanting to travel by water to and from Sydney's central business district.

Patronage on ferries plying the Parramatta River surged by almost 18 per cent to 259,260 passengers in July from the same month a year earlier, the latest figures from Transport for NSW show. The demand for the city's second-busiest route reflects fast-growing growing populations in spots such as Olympic Park and Breakfast Point.

Demand for Parramatta River ferries has increased over the past year. Credit:Wolter Peeters

Under plans to modernise Sydney's ferries, the government has begun seeking expressions of interest from shipbuilders for four ferries each capable of carrying 150 passengers.

Following selection of the successful bidder, construction of the new vessels is expected to begin next year and the first is due to sail by the middle of 2019. Four of the 11 ferries that now sail between Parramatta and the CBD are chartered.