I have tried to find an official ruling from the Catholic Church on this matter, but haven't had any luck and have only come across others asking the same question. This question is from the angle of an Australian and therefore Australian law and marriage law.
Is it possible to have a sacramental marriage only? And not be married in the eyes of the country's law?
This question is relevant on multiple fronts: not wanting to get 'legally' married because neither of you want to combine financial assets, not wanting to get 'legally' married because you disagree with the country's marriage laws, etc.
Another aspect would be, could you get sacramentally married if you are already seen in the eyes of the country's law as de-factos? Or do you need to get civilly married regardless of your de-facto status?