A high-tech journey could signal a new age of train transport in NSW. 

Sydney has achieved a significant milestone in the development of its transport system as the city's first driverless metro train successfully travelled under the harbour for the first time. 

Engineers tested the new track work on Friday, marking the first of many test runs for the line. 

The twin tunnels stretch under the water from Blues Point to Barangaroo, and form part of the broader Sydney Metro expansion.

Hugh Lawson, the Sydney Metro project officer, hailed the successful trial, saying; “Everything went very smoothly”.

To accommodate the Sydney Metro project, six new train stations were built across the city's CBD, including Crows Nest, Victoria Cross, Barangaroo, Martin Place, Pitt Street and Waterloo. 

The tunnels, running hundreds of kilometres long and up to 40 metres deep, required the clearance of 170,000 tonnes of crushed rock and sediment for the Metro City and Southwest line, as well as 650,000 tonnes for Barangaroo station.

The first half of the Metro City and Southwest line, from Chatswood to Sydenham, is set to open to passengers in 2024, while the second half of the line, from Sydenham to Bankstown, will not open until 2025.

The metro lines are designed to be “turn up and go”, with trains departing every four minutes during peak periods, and more capacity expected in the future. 

During the initial testing phase, the trains will run at 25 kilometres per hour, with engineers eventually aiming to trial top operational speeds of 110kph.

To ensure the safety of the system, engineers will utilise an unconventional method during the next stages of testing. 

“We need to know what it’s like with passengers on board,” said Mr Lawson. 

“We can't put real passengers on, so we will have masses of water bottles on board… they simulate the weight of having a fully loaded train.”

The Sydney Metro project has experienced major budget blowouts, with the cost of the project estimated to have blown out by billions. 

The initial estimate for the project's cost was $12.5 billion when it was announced by the previous state government. The cost has since come in at over $20 billion, according to the new government. 

Last month, Premier Chris Minns announced an independent review into the City and Southwest Metro after claiming that the Coalition hid the true cost and timeline of the project.