Taboo. If we're talking about something deeper than civic nationalism - if we are talking about nation-as-a-people, then any group professing such would find itself potentially running against laws both federally ( Racial Discrimination Act, Racial Hatred Act) and state-level equal opportunity legislation.
Federal legislation only covers things done in public, but this has broad enough interpretation that for instance slagging off on some ethnicity in a university, in a locked room but it's easy to hear from the corridor, could be a "public place". Note the internet is a public place and thus all comments you see on chans etc by australians are 60% in violation of the law
One of the greatest challenges for any movement is generating momentum. Nationalists in Australia have not had the benefit of continuing momentum that say European nationalist organisations have had. Momentum is one of the most important things in an organisation. In the business world this is described as "flywheel loop" and the opposite is the "doom loop", I recommend reading on them.
Older nationalist movements have largely fizzled out, become co-opted, or just run a few rallies here and there but never really do much else.
The more recent nationalist groups are largely piggybacking off of america. This is not a good thing for a successful group given that america is a dumpster fire nation. For instance, all of its political dissident groups are largely LARPers who want some idealised representation of the past but have little in the way of pragmatism to get there. I.e. they're ineffectual whiners. History is great but I think to truly succeed, a movement needs a message for the modern day, actions for the modern day, and a vision that looks forward.
What else makes the aus nationalist suffer? Some points I'd put forward would be the tyranny of distance, individualism, modernism in general, intra-group demographics.
Currently some groups of Aus nationalists have a solid intake of people rejected by society. While often educated, these sorts lack organisational/leadership finesse, communication skills (eg emotional intelligence, empathy). They may have issues with trust and or acting in a manner that builds trust (eg holding one's word).
It's not all doom and gloom for such sorts; they're a solid shitposting bedrock and have excelled at online antics. But, the path forward needs things like teamwork and leadership. Even if its in the context of decentralised ad-hoc shitposter gangs, you still need things like that.
Information warfare is probably where the australian nationalists will build their competency next and it could get pretty interesting. The general populace is becoming uneasy with modernity and plenty will be receptive to the first well-presented information salesman (ie propagandist) that comes their way.