Unlimited internet plans have no download limit. They are not capped. The speed is never decreased.
Your speed is decreased when you reach your connections data allowance limit. your connection is reduced to dialup speeds. They do this because if they stopped your connection, they couldn't advertise broadband as being an 'always on' service.
As i said. THE ENTIRE COUNTRY are on capped broadband plans. just the same as yours. Telstra are the only sharks that advertise it in that fashion because they are the only ones who don't use service caps on certain plans. Telstra makes millions because people are too cheap to pay the extra $20 and get a capped service. With the "unlimited" telstra services, you pay about $150 per gig over the data allowance.
I have an ADSL1 1.5mb/s connection that has a data allowance of 60gig. after that 60 gig, the speed is decreased to dialup speeds. My service is not unlimited. because it is decreased after the data allowance.
unlimited plans arn't decreased after the data allowance. in fact, they have no data allowance limit.