Until the contract is settled the buyers have no right to attend a meeting or vote, because they are not the owners. The current owner could give them their proxy but that is up to them.
Note this situation is dealt with in the standard contract of sale. If the special levy is struck after the contract date, the purchaser is liable. Refer clause 23.6 of the standard contract.
A number of adjustments are made at settlement, such as council rates and regular levies.