Confirmed YES you can change from class 457 visa to a tourist visa. Someone did it before. Scroll to last posting at bottom of page in link.
However you have to apply for the change, not just combine it by yourself. If approved you would not need to fly out and return to activate it.
Per Department of Immigration and Border Protection’s (DIBP),
If you want to stay in Australia longer than your original visa lets
you, you will need to apply for a further visa of the same kind or
another visa. How much longer you can stay in Australia will depend
on the visa that you have, or the visa that you apply for. If you
have a ‘No further stay’ condition on your visa, you cannot apply for
a new visa while you are in Australia.
To change the type of visa you have in Australia, you will need to contact the Department of Immigration and Citizenship at least 14 days before your current visa expires. You might be granted a new visa if you fulfil all requirements and you have followed the rules of your current visa. Source
In reality immediately you make that application, you would automatically be deemed to have applied for a bridging visa so you maintain legal status while your petition is adjudicated. You should however still be in good status with your current 457 to be eligible.