Members of the Rail, Tram and Bus Union (RTBU) are refusing to issue or check tickets, with entry and exit gates wide open at major stations.
The union is planning further industrial action over the next 12 days, with a total shutdown possible on Wednesday of next week.
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But RTBU spokesman Alex Claassens says the strikes are planned to avoid inconvenience to commuters.
A four-hour strike is planned for Thursday of this week, when drivers and guards will stop work between 10am and 2pm.
A second fare-free day is planned for next Monday, August 16, but Mr Claassens is hopeful for a resolution of the dispute before then.
But if negotiations break down, the union plans a 24-hour strike on Wednesday of next week, just three days before the federal election.
The union is seeking a five per cent pay rise and an assurance the increase will not impact staffing levels.
