It is important that voters express multiple preferences when voting. If few voters express multiple preferences there is not enough information for any vote counting system to arrive at a a good democratic result. Too many votes can be wasted (become exhausted, in vote counting terminology). A vote for a candidate who has insufficient support to be elected cannot be used to help elect any alternative candidate because no information on further preferences is provided. More subtly, but just as important, a vote for a candidate who has more than enough support to be elected cannot help elect any additional candidates.
Despite the problem of vote wastage due to exhaustion, it is also important that expressing preferences is not too arduous for voters, and voters are not penalised unduly for making mistakes. It is absolutely unnecessary for all voters to express preferences for all candidates. Much smaller numbers of preferences are needed to make this form of vote wastage of little significance. Compulsory marking of all preferences can actually increase vote wastage due to mistakes rendering votes informal, and also forces voters to express preferences beyond the point where they really care and, in most cases, beyond the point where they would actually be considered in a count. It is a waste of time and effort for voters, scrutineers and polling officials.
We recommend that voters be encouraged to express as many preferences as possible. However, they should not be instructed to mark any more preferences than the district magnitude (for example, instructing voters to mark all preferences should be avoided). Furthermore, any vote with at least the first preference unambiguously indicated should be treated as formal, as done in the ACT, Eire and Malta. Preferences should be used up to the point that they are no longer expressed unambiguously. There are ample precedents for the instructions to voters being more stringent than the actual formality provisions (Victorian voters experience it in Senate elections, for example).