The reason most likely is:
Someone at Optus saw a huge number of philshing emails pretending to be Optus and it contained bit.ly links to redirect to the scammers fake Optus page.
So someone decided to block the whole domain without realising it is used for very legitimate purposes and will cause issues if blocked.
Note: blocked message says to only log into your account via the Optus website as it was expected that a person would click the bad link and be saved by being sent to the blocked page instead of bad page.
The net is slowly becoming more authoritatian. The US just repealed 'net neutrallity' (I'm not a sure its ever been an actual thing here). Governments are not the only ones blocking websites, browsers, ISPs, operating systems, google, etc. are all proactively 'protecting' us. This is not necessarily a bad thing IMO. With computers becoming pretty much interconnected what some idiot (or unlucky dupe) has happen on their PC can now affect millions (or a million unprotected PCs can now be put to use affecting others).
But that aside, these blocks are usually quasi automatic. If a spammer / virus etc. uses a IP range or website heavily then the address will be blacklisted. Many companies just subscribe to these lists and so you get the message you're getting. Obviously a URL shrinker site is very open to getting blacklisted for providing links to bad content. Its likely after a while though the ban will be reset.
Have you tried emailing Optus to get the ban lifted as suggested?
Yeah this is a joke. This has sent our business in mayhem this morning. 100s of links needing redirecting, clients pissed because patrons haven't been able to access ticket links to purchase. $1000's wasted on online marketing campaigns over the last 24 hours. How can Optus make this decision without consulting with customers or at the very least warning them?