Thousands of Australians could be disenfranchised at the looming federal election because of changes to the law.

At previous elections eligible voters had seven days to ensure they were correctly enrolled, from the time the Governor-General issued the election writs.

But the federal Electoral Act has been changed and the rolls will now close at 8pm on the day the writs are issued.

The Government has been criticised for making it more difficult for first-time voters to enrol.

Many of these first-time voters are those most affected by the Government's controversial WorkChoices legislation.

But those already on the roll have a slightly longer period of grace - three days from the issue of the writs - to ensure their details are correct.

Spokesman for the Australian Electoral Commission (AEC) Phil Diak told AM in 2004 there were 423,000 'enrolment transactions' processed by the AEC between the issuing of the writs and the old deadline for the rolls to close.

"Of that there were 78,000 new enrolments and an identical number of re-enrolments - people getting back on the roll - and the balance was people changing their address details," he said.

The commission has tried to minimise the number of people who will miss out on being enrolled because of the shorter time periods before the rolls are closed.

"The AEC can't foretell what will happen," he said.

"What I can say is that we've been running a fairly substantial advertising promotional campaign, as well as operational initiatives such as doorknocks - up to a million people have been doorknocked this year - and that's been aimed at communicating the message well before the election, that if you're not enrolled you do need to get on the roll and/or update your details.

"So what has happened this year is that there's been around 375,000 enrolments since the 1st of January, up to this point of time, and that is a greater figure than enrolled in the last election year, 2004, from the 1st of January up to when the writs for the election were issued."

Prime Minister John Howard could make the drive to Government House any day now. But the Governor-General does not issue the writs immediately.

"Over the past 20 years it's been between one or four days after announcement," Mr Diak said.

"So for anyone that's not enrolled at this point in time, or not sure that they're enrolled, and they can check through our website or calling our hotline, they need to do that straight away and get an enrolment form from a post office, or from our website, and return it to the AEC."

Even if the Prime Minister called the election on a Sunday, Mr Diak says people could still enrol before the cut-off.

"For people that are not enrolled, should the election be called this weekend, they could download a form from our website and complete it, and enrolment forms do need to be hand-signed, and you could scan it and send it into the AEC that way," he said.