It's a cyber attack, a denial of service attack.
https://www.abc.net.au/news/2020-03-23/mygov-website-down-centrelink-massive-queues-coronavirus/12080558"The Federal Government has confirmed the MyGov website, which is used to access Centrelink services online, was hit by a denial of service cyber attack this morning.
A denial of service attack is where hackers flood a website with users, so that the site becomes overloaded.
It's the type of attack that was used to bring down the Census website in 2016."
