What do you want to do?
How long are you going?
Business or Holiday?
Do you need to recieve calls or just make them?
Calling to a few people (friends) or lots of unknown people?
What phone do you have?
Are you on a post paid Optus Contract?
What sort of data (Gb) do you want while away?
Probably easiest to buy an Optus Travel Pack over the time involved then. $10 per day gives you unlimited calls and around 1.5Gb of data to use.
Buy them in the Optus App or online here.
Tip 1: Reset the data usage counter on your phone when you leave (and set a data limit of 1.4Gb if you can)
Tip2: Download a local copy of google maps for the regions you are going to. They work when you are offline and no data downloaded.