Some airlines use Forrest as an adequate so that they can fly to Perth non-etops. There is no way anyone is going to land at Forrest single engine even if over the Bight. It will be ADL KGL or PTH.
I think you would have a hard time justifying that decision if it happened at the furthest distance out from KGI and ADL. If you have nominated Forrest as an alternate, then fly past it for another 1.5 hours plus on one engine, when the checklist says land at the nearest suitable I would suggest you may come in for some scrutiny.
Obviously if there wasn't much in it then ADL or KGI is the better option but if the engine fails in the worst possible position then it will be tough to justify pressing on. The other consideration is if you press on then something else goes wrong that leads to an accident you are going to get smashed in an inquiry or court case.