Once you get off the fire trails, it is pretty thick scrub, even on the top of the ridges. It's not impenetrable, but sometimes takes longer than you expect.
On one trip, I started up the fire trail where you left your car to get to the top of the ridge, intending to follow it back alongside Oyster Shell Road to the main road. Tuned out to be very scrubby. Ended up having to drop off the side about 1km from the end as it was getting a bit dark.
It was pretty treacherous getting down almost a cliff, but even worse, we ended up in the back yard of a property with rottweilers as guard dogs. Fortunately, the owner was there and kept them under control.