While some of the growth may have come from agencies shifting away from GovSpace, which shuts down next year, part is also coming from state, territory and local governments who are beginning to consider the platform seriously.
While mandating a single webCMS and platform might be a step too far for Australian governments, the approach of providing a cheap and effective platform, with full standards support, a growing developer base and interoperability of plugins and modules (which can be reused across agency sites), is providing a strong 'pull' effect.
This 'pull', rather than a 'push' (mandated) approach to service design is one that government can also apply to citizen and business services, so I'm hopeful that the GovCMS experience is demonstrating to agencies how the carrot can be more powerful than the stick.
Given that even the Digital Transformation office has now fallen into line, after the DTO initially considered building its own WebCMS for the Gov.au site, GovCMS has been a massive success for government in Australia, and for the Department of Finance in particular.
GovCMS is supported by Acquia, the commercial entity created by the developers of the open-source Drupal platform, with a variety of local development partners involved in the development of specific agency sites.