Optus $2 Days prepaid plans have been around in various forms since 2010. The plans were recently updated to include access to Optus' 4G network, and in their current form might represent the best-value prepaid deal for data-heavy Optus customers.
Here's how the plans work. As with any prepaid plan, you pay up-front for credit. You can recharge with $10, $20, $30, $40, $50, $70 or $100. Credit expiry periods match the dollar value of the plan (so a $10 recharge will expire after 10 days).
On any day where you use your phone, you're immediately charged $2, but you can make unlimited calls and texts to Australian numbers until the end of the day. You also have 500MB of data on offer. All of that is covered in the single $2 daily fee.
If you don't use your phone at all for outgoing calls or data, then you're not charged anything. Note, however, that to take advantage of this you would have to make sure push mail and any other services were switched off -- even a single data download will incur the fee. On modern smartphones, this seems a fiddly way to operate, and it's probably safer to assume you will use the credit every day. (The day is calculated from midnight Sydney time, which might make life slightly trickier in other timezones if you're hoping to have occasional 'free' days.)
Calls to international or special rate numbers are charged directly against your credit. If you exceed 500MB of data in a day (not impossible, especially if you're tethering), you're charged an additional $2 and given another 500MB. If you use all that as well, you're cut off entirely for data access until the next day begins. (Using 1GB a day on your phone seems unlikely unless it's your sole source of net access).
Because of the $2 a day charge, you're very unlikely to reach the expiry date ($10 credit has a 10-day expiry but will only last five days if you use it every day). If you recharge before expiry, existing credit rolls over.
What this means is that over a 30-day period, you would have 15GB of data plus unlimited calls and texts for a cost of $60. There's no other plan on the market offering that volume of data on a 4G connection, prepaid or postpaid.
In our recent roundup of the best prepaid plan on each Australian network, we picked Amaysim as the best option using the Optus network. Amaysim's unlimited plan costs a fixed $39.90 a month and offers 4GB of data plus unlimited Australian calls and texts, but doesn't offer 4G.
If 4G access on Optus matters to you and you're sure you'll need more than 4GB of data, then $2 Days is the clear choice (and a better value option than the Live Connected option we recommended when originally assembling that article). That said, if you're not going to use 4GB of data, or don't often have access to 4G coverage, paying the extra $20 a month doesn't seem particularly worth it. As ever, your own usage patterns are your final guide.