The privately run service, which costs an adult $10.20 for a trip between Circular Quay and Manly, will be included under the $50-a-week Opal cap from mid-2023.

Peak services will be boosted to six an hour, creating a turn-up-and-go service for those travelling the picturesque route.

"There might be a green (public) one there or a blue (fast ferry) one - it doesn't really matter. They're all going to be part of the same product," Transport for NSW chief executive Howard Collins told reporters on Friday.

"The big winners will be those from western Sydney who have taken the day trip to Manly as an important part of their summers," Transport Minister David Elliott said.

The 15-year contract extension announced on Friday contains provisions to convert the ferries to hydrogen-diesel hybrids in four to five years, subject to regulatory approvals.

The new fuel mix would use hydrogen as an accelerant, reducing the quantity of diesel needed. The first iteration should cut carbon emissions by 50 per cent, with an aim of an 85-per-cent cut by 2038.

Plans are afoot to provide a fully electric service by 2038.

Mr Elliott thanked officials for burning the midnight oil to complete the deal with the service's operator, NRMA.

It's the fourth announcement in as many weeks for local member, Environment Minister James Griffin, who faces a challenge from Climate 200-backed teal independent Joeline Hackman.

Ms Hackman welcomed the Opal deal but said NSW was a laggard when it came to electrifying ferries, compared to Canada, New Zealand, Denmark, Sweden and Seattle.

"Some of these locations have been actively upgrading or ensuring that all new fleet purchases are electrified for the past four to five years," she told AAP in a statement.

Mr Griffin sidestepped a question on whether the new discount was an attempt to curry favour but said the fast ferry sitting outside the Opal fare system had been raised by voters for "many, many years".

"What we're seeing is people wanting more reliability, more frequency to get to and from Manly and this ... does exactly that," he told reporters.

"It will deliver a great outcome for the environment and help us get on our way to net-zero emissions that this government is proudly pursuing."

The deal to pull the fast ferry under the cap won't result in the government subsidising the NRMA, the government says.

Instead, both parties will share funding and profits under a "pain-gain" agreements, depending on commuter numbers.

"Over the 15 years, the predictions are that there will be money flowing back into government and less charge for our customers in overall tax," Mr Collins said.

It follows the government's plagued replacement of 1980s-era Manly ferries with the faster, lighter Emerald Class fleet. Some workers suggested they would be better as "diving wrecks".

Labor said the new Opal plan was an admission the introduction of the Emerald Class had failed.

"It was a dud deal from the beginning, they have never really worked and this is them trying to clean up their mess about 80 days before an election," acting Labor leader Prue Car said.

Australian Associated Press