The only entitlement to a break is that you must be given a 20 minute break if your shift is 6 hours or longer. (There's no 'pro rata' provision with that entitlement. If you work a 12 hour shift you're still entitled to just one 20 minute break, not two).
The break can't be at the start or end of the shift but it doesn't have to be in the middle. So, for example, someone working a 12 hour shift could be given just one 20 minute break after 11 hours continuous work. That would certainly be 'bad practice' but it wouldn't be illegal.
Chris