You may have noticed most, if not all, basic monthly prepaid SIM plans have had a price hike within the last year. For example, mine went from $10 a month to $15 a month. I’ve been shopping around for an alternative and, with the EOFY sales on currently, the annual prepaid plans are a significantly cheaper alternative than the monthly plans (often with more data too).
The cheapest I have found so far is Catch Connect’s 60 GB plan which is currently discounted to $89. That works out to ~$7.42 per month, with 5 GB of data per month on the Optus 4G network. If that’s not enough data, their two higher tier plans work out to ~$9.08 and $12.50 per month over a full year with way more data.
Other options you may want to look at are Kogan (Optus 4G network) and Lebara (Vodafone 4G network), though neither have an option as cheap as Catch Connect. There may be differences in how your data allowance is provided (up-front or monthly) and whether rollover is included so check that out as well.