Obviously only work with phone that have eSim.
Buy a $30 Optus prepaid epic data 35GB pack for $10 online at Optus here
Activate as usual (use your own debit/credit card for easy id check, you can disable auto recharge), create an account and download My Optus app login make sure everything works. check you data and expiry should say 35GB and 28 days. Make sure sure you can receive sms at this point and everything is done on the phone you want to port the eSim to your physical sim is also on this phone.
Now go here and choose - replace sim - eSIM - Continue - Continue - and follow the rest of the instruction. It say could take upto 5days but the whole process from activating sim (porting in old number to activation of eSIM) take less then 10 min.
Once step 3 all goes well you should receive a sms contain the link to download and activate the eSIM (it will need the App so make sure to do that in step 2).
Just follow the instructions, it self explanatory at this point. once the eSim is activated you will see no service or sos on the other line. You can now remove the physical sim.
Restart device, log out and back into the apps and check the data, hopefully it will show 70GB with 56days expiry.
[the eSim online activation has been disable]