As pax on SQ, SIN - BNE one night, we diverted to Townsville due early morning fog. But we diverted enroute, without getting anywhere near BNE.
Coolangatta (now Gold Coast Airport) doesn't suffer fog nearly as bad as BNE. Being only a few minutes down the road and on the way to SYD anyway, it could be worth a look to save going all the way to the big smoke.