I am currently living in Italy on an Australian passport and a 5 year permission to stay document from the Italian authorities. As such can I travel to Portugal for a few months???
When you have a residence permit for a Schengen country you can travel freely within the Schengen Area without needing an additional visa. The 90 days rule does however in theory apply.
Since border crossings within the Schengen Area are not registered in practice the 90 day rule is not enforced. However you will need to carry a valid ID (your passport in this case) and your Italian resident permit with you.
The issue to be aware of however is that if you stay somewhere longer than is usual for just a visiting tourist you are expected to register as a resident. In Portugal I believe this is after 3 months, and even EU citizens need to register. You will need a Portuguese residence permit for that.
So for how long are you intending to visit? If it is two months I would expect no issue. If it is like 9 months you may get issues if this is discovered by the authorities.