Put the phone / charger/ cable inside a bowl of rice and seal it up (Glad wrap? Zip Lock Bag?).
That should extract any residual moisture in 6 hours or so. If you're still getting the error then it would seem to be a fault and you should ask Optus for an exchange or repair. Can't say if the phone or the charger is at fault. Can you try a different charger?
We are able to send accessories (including the charger) back to the manufacture for assessement. If they do deem the device faulty, they'll replace the charging brick under your standard warranty.
Water damage isn't covered under your warranty, but if you believe the charge hasn't made contact with any liquids, then you can definitely request an assessment.