Base charge and over charge is far more sensible. The end user should never be penalised. Basically ISP pay for a base load at one rate and anything beyond that load is charged at a higher rate. The ISP can save money by pre booking more but the end user is not penalised for the ISP getting it wrong, it just means the ISP loses profit when it goes over.
How about when it is impossible for the NBN to supply what they are claiming to sell on their net work, when they over sell CVC beyond hardware capacity, the ISP pays, the customers pays even more and the NBN enjoys legalised fraud.