TV channels getting "hacked" seems like it's going to be more of a thing in the future.
A couple of years ago the French TV channel TV5Monde (a channel aimed at the world, and offering French news, documentaries and dramas), was hacked in what was almost certainly a state sponsored attack.
And TV networks, especially in news environments, are run from big complicated playout systems where story packages are ingested and then played back out. Nobody is rushing around with big tapes like they're on Broadcast News anymore. So it's not hard to conceive that if someone took control of that system, they could massively disrupt a provider's output. I suspect that cyber security teams in networks around the globe will be looking very closely at what happened at Nine and making sure they're locked down as much as possible.