Please consult the "Mobile Carriers" forum on Whirlpool, they have this question asked over a dozen times a month with the detail. Please note, the offers that Optus has can be discussed here. However you wont get anything cheaper than $30 per month.
Optus long expiry plan. $60 for twelve months:
I use such mobile plan and happy, because It is cheaper than previous one. I like to call to my all intarnational friends and stay in touch with others. Btw, two days ago I got message from unknown number. message had propose to donate money to one organization. I checked that number and found that the area code +56 is the country code of Chile. I used this service https://areaphonecodes.com/chile/ I deleted that scam message.