This is one of the biggest misconceptions when it comes to permanent residencies. When you get your PR, you will be given a 5 year travel authority, which can be renewed in two ways, either by staying in Australia for 2 years out of 5 years, or if you have been in Australia for less than 2 years, it renews for 1 year at a time.
In fact, as long as you get back to Australia before your initial 5 year travel authority expires, you can actually live in Australia for the rest of your life (that’s why it’s called permanent residency) without having to renew anything!
You only need to renew your travel authority if you want to leave the country.
For provisional residents (subclass 491) you have to fulfil specific stay conditions, so it does not follow the explanation above.