All NBN Satellite services are calculated within a rolling 28 day window. This can sometimes be confusing as many internet services are based upon a monthly data allocation.
A simpler way to explain this is to ask the question:
Over the past 28 days how much data have I used? Is it less than 24 Gigabytes of upload or less than 150 Gigabytes of both upload and download?
The trick is that you are required to ask this question every day.