The download limit mystery has been solved!
Mr CurlyPops was watching the Rip Curl Pro online on his laptop over the weekend.
We're on a 5GB plan and there's only 2 people in the house so we couldn't possible need to upgrade.
Thursday had double normal usage. Friday had 4 times normal usage. Saturday was 3 times normal usage even though he went to Bells Beach and watched it live!
Lesson learnt.....don't watch tv on the internet!