Don’t know if you venture out from the city much but seriously if you do Optus lasts about 50km and then you get them at the next big town , whereas Telstra have a LOT more mobile towers.
A good example is between here and my old home town of Moree.
My wife has an Optus prepaid phone ( uses it rarely other than to make phone calls as she has an IPad which she prefers ), we leave Brisbane and head to Toowoomba , her phone does get some coverage , mine full bars of 4G.
We use the new Toowoomba bypass, her phone kicks into gear as we pass Toowoomba and then cuts out for the next 200kms to Goondiwindi, I have service all the way bar about 20 kms.
From Gundy to Moree ( 110kms) here cuts out 5 kms away and then 5kms to the two towns , mine remains as 4G the entire way.
Now that’s Optus , the second biggest coverage in Oz, if you have anything else it’s even worse again.
Telstra , if you travel , is the ONLY option.