Draft state government transport plans include removing express trains between Cronulla and the City, and replacing them with high-frequency, all-stations services.
More express trains would be introduced from Waterfall.
The draft plans also include constructing a new train line between Kogarah and Parramatta - a proposal first unveiled in 2018 by the former Liberal government in The Future Transport Strategy 2056.
The Sydney Morning Herald reported the "secret" documents, which were being worked on by senior transport officials, included plans to extend the "inner-urban network" from Hurstville to Cronulla, which would allow for a "single, all-stops, high-frequency stopping pattern".
While commuters from Cronulla "would no longer have access" to express services, "this will be offset" by more frequent services and new digital systems, further reducing journey time impacts", the documents were quoted as stating.
A spokeswoman for Transport for NSW said, "The Future Transport 2056 strategy, published in 2018, first identified a possible strategic long-term rail connection between Kogarah and Parramatta.
"This has not been costed, and no other work has gone into it.
"It is the role of Transport for NSW to develop long-term plans for a growing city like Sydney to ensure the public transport system grows and evolves with the city.
"This draft plan has not been approved by the NSW Government, and none of the new lines on this map have been costed yet."
Opposition Leader and Cronulla MP Mark Speakman said the plans for the Cronulla services were outrageous".
"Work was progressing under the NSW Liberals to increase peak hour express services to and from the Cronulla line by 30 per cent," he said.
"Now, instead, the Minns Labor Government has been caught out with secret plans to axe all express services to and from the Cronulla line.
"Instead of improving infrastructure to match increased housing density, Chris Minns wants to do the opposite. He's already making life much harder for commuters struggling on public transport and our roads."
Mr Speakman said the idea of a Parramatta to Kogarah rail link had been a long-term proposal for some time and had strategic merit.
"However, the Minns government has pretty much ruled out doing major transport infrastructure projects in the future, with one of its ministers, for example, calling the Liberals' metro plans 'vanity projects'.
"These sorts of projects will remain just lines on a map under Chris Minns, who lacks the vision to help struggling commuters and match growing populations with more public transport."