Let me see what else I can find out about the old railway station at Helensburgh. The line opened to Helensburg in 1888 with the intention of continuing on to Illawarra. It was only used for less than three decades and closed in 1915. Slowly the platform became overgrown and started to disappear, but at some point volunteers cleaned it up.
I think this the Helensburgh Tunnel (no. 3). It is a short one, 80 metres long and one of a number in the area. Some of the other tunnels were nearly a kilometre long and had very steep 1 in 40 grades and it was not unknown for a steam train to stall in a tunnel, whereby it would return to the previous station and offload some carriages and have another crack at getting through. Now imagine if you were a passenger, or train crew, crawling along or stalled in a narrow tunnel with the steam engine belching out smoke, soot and steam. A less than successful ventilation shaft was added to the Metropolitan Tunnel (no. 4) in 1891. Later a monster steam driven blower was installed at one end of the tunnel which effectively cleared the smoke and soot.
The most famous disused train tunnel in the area is the Otford tunnel (no. 7), one and a half kilometres long.
The line became too busy for single track operation so new tracks were laid on a different route between Waterfall and Coalcliff. South the line continues on to Illawarra and if you catch it north to the end, you will be at Bondi Junction.
These photos came from Tumblr and I don't know whose they are as they were reposted there, so I can't credit them.
This appears to be the old railway station platform. I doubt the sign is original. A short length of track leads into the tunnel.
This must be the other end of the tunnel, somewhat less overgrown but it could be one of a number of tunnels in the area. Any corrections welcome.