If a Frenchman was to commit a French crime while visiting Australia, where that action is not a crime, can France prosecute them for that action?
That depends on the details of the French law involved. If the French law says that it applies outside of French jurisdiction, that it may.
There are, I know, US laws that specifically apply tom actions by US people that occur outside the US. For example the The Foreign Corrupt Practices Act (FCPA) at 15 U.S.C. §§ 78dd-1, et seq. makes it a crime to pay bribes and make improper payments outside the US. The US does prosecute its citizens on charges of violating the FCPA. There could be a similar law in France.