Worse, we can't rely upon techniques equivalent to financial auditing to detect that an election has been tampered with. It is true that local government voting papers identify who cast the vote, so it would be possible to ask voters if what was recorded was what they meant. But how many voters will need to be audited to detect cases where just enough votes were changed to change the electoral outcome? Maybe so many that we might as well have run the election manually.
Another issue is that the devices voters use may not be secure. In the US, up to 30 per cent of computers contain malicious software that could prevent ballots being transmitted, or replace them with entirely different votes on a mass scale. That isn't possible with a postal voting system, for example.
A third issue is that electronic votes cannot be counted or recounted, if results need to be audited, by individuals or election observers. Checking actual ballots is a lot more reliable than checking computer systems for signs of tampering.
All this means that if we can't guarantee the integrity of an online voting system we can't trust it - and that puts the whole democratic process under threat.
Proponents say online voting might pull in apathetic younger voters who live their lives on their screens, or increase voter turnout. Except it might not.
Evidence from Estonia, the first country to introduce online voting for its parliamentary elections in 2007, suggests internet voting simply makes voting easier for those who already vote. It had just a 1.5 per cent rise in overall voter turnout between 2003 and 2007, and only 30 per cent of the population use it even after a decade.
Running an online voting trial for local body elections and potentially providing a pathway to online voting for national elections before these problems have been solved is a dangerous exercise that could ultimately destroy trust in our electoral process.
Academics are researching promising techniques based on advanced cryptography, but it may be decades, if ever, before we can vote online with confidence.
• Ian Welch is an associate professor in the school of engineering and computer science at Victoria University of Wellington.