Curiously enough, the proponent of this policy, Evan Thornley, is not a religious right-winger, but a member of the Fabian Society, that very Britishly pragmatic socialist organisation which once had George Bernard Shaw as one of its members (and, during the Cold War, was accused by Bircher types of using its shadowy influence over the Labor Party to implement "Sovietisation by stealth").
There are, of course, numerous problems with this proposal. Were it to be adopted, politicians would start bidding for the votes of large families by giving them more money, taken by punitively taxing the suddenly all-but-disenfranchised non-breeders. (What are they going to do, vote for someone else?) This would result in a system which effectively regards not having children as deviant behaviour to be penalised; once this is a matter of bureaucratic fact, the culture would soon follow. And then there is the likelihood of a bias towards large families bringing with it a bias towards religious conservatism; all of a sudden, Victoria would look like the repressively paternalistic 1950s white-picket-fence dystopia John Howard didn't quite succeed in building.
Of course, that's if such a policy were ever adopted. There are practical problems with implementing it, such as deciding which parent gets their childrens' votes. Granted, they could be split in half (with each parent in the 3-child family having 2.5 votes), though this proposal effectively changes the paradigm of democracy, from one comprised of voting individuals to one comprised of voting families. It has echoes of the top-down "strict-father" model of the family so favoured by conservatives, and at the heart of the culture war in America and Australia: it reinforces the idea of a family being defined by a chain of authority residing in the head of the household. Granted, it does not define a head of the household, though it is a short distance from accepting the paradigm that votes are allocated per household, and not per individual, to accepting that the votes for all members of the household are cast by the head of the household.
Mind you, given that Thornley's boss has suddenly resigned, this proposal is likely to be even more dead in the water than it was before. Unless the Howard government decide that it has battler-rallying potential and put it to a referendum, or else Rudd decides to use it to outflank the family-values warriors on the right.