If you are an Australian citizen, you can explore the USA E3 work visa route which is only available to Australians. Via USA you can explore Canadian job market too while still sticking to your passion. There are lots of design and research opportunities within academia which need engineering background with software assisted expertise. Firms which build industrial design software are always looking for talented engineers like yourself.
Similarly, Australian citizens have easy working holiday visa options within Britain.
Even if you crave for coming back to Australia in future, your experience gained in international markets will place you above the competition.
Additionally, don't just confine yourself to just USA and Britain.. Must look at anywhere quality design jobs are available (Germany, Japan, Israel, China, India, Nordic countries, Russia... yes Russia is way ahead of the rest when it comes to Mathematics and applied engineering areas.. Don't go by the media hype, use your own rationalization).
My suggestion will be to stick to your core expertise which is mechanical design and anything which leverages your engineering degree. Almost all areas of modern businesses need software skills too. Your engineering background will be highly sought if you use that as your main asset plus hands on knowledge of design software.
Never stray away from your passion due to external world's influences. Software development ultimately is a tool serving core business use cases where domain expertise and core engineering knowledge are still highly valued. If you join as a Java or C++ developer in an unrelated software firm, you'd be treated as a normal developer. Joining a design firm as a mechanical design engineer with software skills will place you as an expert right from the beginning. That will give you an edge while negotiating your future salary.