You're definitely dredging up some memories of running through 50 metres of drainpipe with my head ducked (it wasn't quite tall enough for me). I assume it's still there (don't think anything's changed with that part of Hindmarsh in the last 28 years) - if so head for the bottom of the gully and you'll find it.
Thanks Blair! I found it this time though lucky I found this comment because I couldn't figure out where it was on a map