Every day the population of Australia grows by about 700 people. Every two minutes there is an extra Australian. At least, that is, if you believe the Bureau of Statistics. But according to the Australian Electoral Commission the population of Australia is in decline.

In April 2005 there were 13,216,525 Australians on the electoral roll. In December 06 there were 13,174,866, a fall of around 41,000. If it had not been for a surge created by the Victorian state election the numbers would have been much lower.

In Victoria the numbers on the roll fell 32,000 or so over a 17-month period before rising about 67,000 at the end of the year.

Well Victoria is rustbelt territory, some people would have us know. What about booming WA? It seems the West is in decline as well, losing twenty thousand voters in 18 months.

The population of Australia has grown by about half a million in the last two years, but that hasn’t shown up on the electoral roll. Unless the situation gets fixed, come election day half a million Australians are going to miss out on their chance to vote.

The Victorian election appears to provide some good news – putting on those thousands of extra enrolments in three months as the election came round – who cares if people are not on the roll when there is nothing to vote for?

However, looking closer, things are actually even worse than they seem: 26,000 of those people added to the roll as a result of the election got added too late to vote. Mostly they are people who turned up at the polling booth, found they were not on the roll and filled out a form then.

There is a crumb of comfort that they will be ready for the federal election, but their equivalents in other states may not.

Even worse, a great many of the forty thousand who did get on the roll in Victoria in time to vote only did so because they had plenty of warning (fixed term elections) and could do so after the election campaign hit full swing.

At the federal election this year that won’t be the case. Changes to the law mean that anyone not on the roll has one day after the election is called to enrol. Not many will.

Half a million voters is 3,000 per seat. Enough to make a pretty sizeable difference to the election result one would think. Of course, if the voters who have been kicked off, and the new ones not added, were a random selection none of this would matter, except in a philosophical sense.

But if past experience is any guide the missing voters are not random – they’re young people who’ve just turned 18 and renters who have moved house (the two groups obviously overlap a bit). And both these groups are disproportionately likely to vote Labor and Green.

That doesn’t mean that the ALP can look forward to missing 3,000 votes per seat. But it does mean that, after preferences, a few hundred more votes are more likely to be missing from the Labor column than the Liberal.

Presumably the more astute Labor candidates are onto this, and are employing people to check that, in marginal seats at least, people are properly enrolled. But if you want to know why there is a high chance the Senate will stay in Liberal hands, no matter what happens in the House, it’s worth considering the collective impact of all the unenrolled voters. And asking a few searching questions of the Australian Electoral Commission.