Returning permanent resident - Visa issues?

I was born in England but moved to Australia when I was a very young child. I am a permanent resident of Australia, but left Australia in August 2010 to move to England for family reasons. My permanent residency Visa expired in 2012 but I believe this expiry is 5 years after the date it was issued (February 2007), but I was led to believe that you can return to Australia on your permanent residency Visa for up to 5 years after the date you leave the country. I want to return, so do I simply reapply for my expired permanent residency Visa or do I have to apply all over again, starting on perhaps a temporary visa? Also, before we left Australia, we had a citizenship application accepted, but instead of pursuing it, we left a few months later. Will I have to reapply for citizenship after a number of years of being back in the country or will the citizenship application still be valid? Hope this all made sense, it's complicated!