It would be great if some of the web sites like mygov or the ATO business portal used stuff like this instead of the brain dead frameworks they use now.
The business portal already has work underway to replace it. The Tax Agent portal has a replacement currently in beta. My gov itself doesn't have much content on it - or are you referring to ATOOnline ?
Centrelink/DHS and myGov went through separate projects to redesign and use some parts of the design system.