Old Metropolitan Tunnel is publicly accessible, with information signs about the history nearby. The glow-worms are cool, so is the unearthed partial platform. The tunnel is blocked at the southern end, you cannot go all the way through it (you must exit the same way you go in). In any case, you'd be up to your balls in mud if you tried as the southern half is almost always flooded (the tunnel was once used as a water reservoir, that's why one end is blocked).
Be prepare to run into other people there, it's very popular.
There are other abandoned tunnels nearby, however, access is iffy.
Blakjak of the Sydney branch of the Cave Clan.