Plain and simple, Digiweb are crap and thiefs
I was with digiweb about a year and a half ago and had a similar problem.
I read the terms and condition but to be honest I wasn't 100% sure what the "rolling" period was.
Anyway, here's how it works........If I use 2gb on the first day of the month, then 4gb on the second and then 24gb on the third day of the month, thats my 30gb allowance used up so for another 28 days the limit my bandwidth to a little over dial up connection.
Here's the good part....you don't get your full 30gb allowance back at the end of the month. This is how it works.....I used 2gb on the first of last month right? So that means I get 2gb back on the first day of the new month then i get 4gb back on the second day and then 24gb on the third day.
You do the math!!! That is only an example, so if you were to use 5gb on some days,10gb on other days basically your screwed if you want to download consistantly.
That part 1 of my story, here's part 2:
So I cancelled crappyweb and they said ok and cancelled my account.
I came home one day to find a slip of paper in my letterbox saying digiweb were here earlier and took the antenna from my roof. Now that pissed me off

I have a skylight in my house and some jackass from digiweb was on my roof with a clear view into my house. And he go onto my house without my permission. I rang and complained and they agreed the guy shouldn't have done this.
Part 3 of story:
I paid this amatuer service a month in advance and rang them and they said they would post me out a cheque. About 2 months pass and no sign, I ring back up and I'm being passed from department to department talking to idiots that havn't a clue whats going on and keep telling me to email them.
I'm now with magnet entertainment and am as happy as a pig in Sh*t :-)
I'm on a 4meg dl connection and 1meg ul and 100gb monthly allowance and yes when the month is up.....then your 100gb allowance is started fresh.
my opinion as you see is digiweb are crap and crooks that still owe me money but I'm rid of them now and thats good enough for me.
Sandisk