I don't know about overseas, but here in the states every time politicians give out an estimate on how much revenue a new tax will generate they tend to be gros

I don't know about overseas, but here in the states every time politicians give out an estimate on how much revenue a new tax will generate they tend to be grossly incorrect at the end of the year. The reason is that if you tax an activity then people do things to avoid paying the tax for that activity; stop, move, or avoid. Sad thing is, the politicians make the revenue announcement so they can go ahead and spend the money as if they've already collected it.