Stretching the memory a bit, but I reckon it was closed around 2000-2002. I can remember stopping there. I do recall the vista itself getting quite overgrown, and
also the tourist buses pulling onto the freeway and crawling up the hill towards Kariong at very slow speeds.
Not sure if there was a particular crash that generated the closure, or just a combination of the risk and the overgrowth being a convenient reason.
With the way traffic volumes have increased on the M1 there's no safe way to re-open it; the bridge is too close for proper deceleration/exit lane.