As a holder of an Australian passport you should be eligible to enter the EU Schengen area member states for up to 90 days.
See this article on wikipedia and especially the map at the bottom of the article.
http://en.wikipedia.org/wiki/Schengen_Area
Consequently, it would appear that, subject to anything that would preclude you personally entering a Schengen Member State, there would be no problem in you exiting Lisbon Airport for your layover as Portugal is one of the Schengen Member States.
