The NSW government has been working with Optus, Telstra, and Vodafone to install more than 10 kilometres of cabling to provide reception for mobile phones, tablets, and laptops.
The service is undergoing live testing, and will be switched on as early as Monday, Transport Minister Gladys Berejiklian said.
"The lack of coverage in our CBD tunnels has been an ongoing concern for customers over many years, and quite frankly an embarrassment for our city," she said.
More than 400,000 commuters will benefit from the new infrastructure upgrade, Berejiklian said.
The rollout will continue on the Eastern Suburbs Line, with work underway to install 18 kilometres of cable between Erskineville and Bondi Junction.