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You might be thinking of the Tomewin Mountain Road from the Currumbin Creek Rd on the Gold Coast to Murwillumbah. Its a great twisty road. From Murwilumbah, follow the signs to Uki, go through wonderfully colourful Nimbin to Lismore, down the Summerland way, turn off at Pringle Way toward Lawrence which leads you into Grafton. I think coffs is just a half hour or so from Grafton. We wnt that way to the Jacaranda festival last year. I have a different route planned for this years run.

Or maybe the Tweed Valley Way which is also a nice detour from the highway around Murwillumbah but dumps you back on the Pac Hwy just a bit further down the road. As you mentioned, I think at least part of it (or maybe all) is the old Pac Hwy. I prefer the way from my last post.

Nerang - Murwillumbah: Route 97 via Numinbah
Grafton - Coffs: follow the Orara Way