Because this is a public forum and we're all customers of PM... nobody here really knows. However, you can always book mark the plans page and check it on Monday's and Friday's. Also, keep an eye on other providers for offers. It seems when they offer something good, PM seems to try to match it or better it. But it does take them a bit longer as PM isn't their golden child and we're like the last to know.
@Kaylup_S - usually you need to keep up with the current inmarket plans to see any new plans being offered. If you are paying more for something being offered at the same data amount, then no one will let you know this; if is on us to do the research and make those changes. So check your plan options in your My Account occasionally.
Perhaps this year Public will offer free data add freebies again, as they have over the last 3+ years. Although, I don't know how they will work if they are expired in 30 days after being added to the account. Hopefully they can make these stay until all used up.