PLEASE DO NOT attempt to access from the M1 - dangerous, crazy, illegal and will not be successful - it isn't up there... Access the cache from the Pacific Highway. It's much nicer down there anyway, unless it's a Sunday and the motorbikes are crazy....
We have a little series around Newcastle, based under/over major roads, but not on them, which we've based on a similar series from the North Coast. We saw this spot and thought it would be a good spot to place one. You should be looking under the faster lanes of the northbound M1. The container is a polycarbonate Pelican type box, so it isn't tiny - room for swaps and TBs. There is an unactivated TB in there for FTF or whomever gets it/wants it first.
There is heaps of room to park just north of GZ. The GPS was a bit sad at GZ, so coords are based on averages from either side - use the hint, and watch your footing, and stay off the road.