Can anyone (in particular a Skinny rep) advise how casual data is charged?

I understand the new rate is 20c/MB, but over what period of time does data usage get collated to be charged?

I found the following in Skinny's T&Cs:

"Skinny's standard data rates are calculated based on usage and billed in 5KB blocks, rounded up to the nearest 5KB at the end of each session or every 15 minutes (whichever comes first), and a minimum charge of 1c per session applies."

Does this, in effect, mean small and infrequent uses of data will be only charged to the nearest 5KB? Or does some rounding occur where data is rounded to the nearest MB? (I'm assuming the former, based on the quote above, but wanted to be clear as to its meaning amd whether I'm missing something from this equation.)

Context: I'm wondering whether it's far more cost-effective for those using casual rates to use data to message (via an app such as Whatsapp) as opposed to paying that grossly inflated sum of 20c per SMS. Of course, even if it's charged at 20c per usage (ie per message sent via a data-using app) it'll be the same cost as an SMS, but I'm assuming from the T&C quote above it'll be only a subset of that 1MB of data (therefore a proportion of the 20c/MB), based on the quantity used in the session or over 15 minutes, which is likely to be a relatively small amount of data if only sending the occasional message.

I understand this isn't relevant for most people, given they're on some kind of plans, but there are still some out there who rely on casual data rates, and will be impacted badly by this significant hike in SMS charges (which, I note, Skinny has failed to explain). My wife, for exanple, has a work phone, so it's really not worth her being on a plan just for the periods where she's carrying/using her personal phone.

Thanks.