When we rebuilt australia.gov.au
in the late 2000s, we used a proprietary system and an outsourced provider. The system was then quite complex as it had to manage the MyAccount function as well as the more general serving of information about Australia, the Government, and its services.
While effective, the system was expensive to maintain and amend, hosting and licence costs were high, and there was very little flexibility. Changes to the system took a long time and lots of resources. It was unsustainable.