I don't know the infrastructure in Oz, but I think you've answered your own question there
Theres no way a mobile phone mast is going to have the available bandwidth to deal with the contention caused by a horde of kids all logging on at the same time. You've been suckered by a marketing ploy.
Solutions?
Go back to ADSL
Shoot the kids
Get a satellite connection (probably expensive.....unless you shared it with others)
Where do you live - in town or country?