The outage apparently began 4am AEDT. It's now 11am. For 7 hours and counting - no network at all in large parts of the country including no mobile network at all (if not all - I haven't heard of anywhere where it is working fine).
The impact is huge. As others have mentioned it's impacting financial services networks and in Melbourne, it's impacting the Metro train network (they use Optus for their backup communications).
The very widespread nature makes me think "there's gotta be more than one failure here" e.g. can't just be a DNS issue. Follow up thought - "could this be a non-accident?" i.e. a deliberate outage on behalf of actors unknown?