Up to 100,000 more Australians could be allowed to vote in the August 21 election after the High Court today ruled that parts of the Electoral Act are unconstitutional.

The advocacy group GetUp! believes tens of thousands of people may have been excluded from the electoral rolls because they missed registration deadlines introduced by the Howard government in 2006.

Before the changes, voters had just seven days after an election was called to enrol to vote. Under the new laws, the rolls shut on the day the election writs were issued.

Two young Australians who found themselves in such a position took their case to the High Court, arguing the changes to shorten enrolment cut-offs were unconstitutional.

In a majority ruling, the full bench of the High Court today declared certain parts of the act invalid.

That will mean anyone who was seven days late with their enrolment will now be able to vote.

The Australian Electoral Commission (AEC) has made plans to ensure those voters will now be included on the rolls for the upcoming federal election.

GetUp! national director Simon Sheikh says it is a historic day.

"Let's remember, there was of course no rational reason why John Howard brought these laws in in the first place," he said.

GetUp's counsel had argued that the houses of parliament would not be chosen by the people because a substantial number of citizens would have been excluded from the vote.

ABC election analyst Antony Green says the decision is likely to favour Labor and the Greens.

"Given all the publicity, I would imagine there would be a huge increase in the number of 18-year-olds on the roll and that would probably assist Labor and Greens," he said.

The AEC says in the coming days it will process enrolment claims that were correctly completed and submitted after 8:00pm on July 19 but before 8:00pm on July 26.

It says it will contact the voters concerned and tell them they are entitled to vote and how to access further assistance.

The lists used at polling booths to mark names off have already been printed and distributed.

The commission says that means any voters who enrolled late will have to cast a "declaration vote" and provide ID such as a driver's licence or passport when voting.

The Opposition says it will be monitoring the situation for any signs of electoral rorting.

Shadow special minister of state Michael Ronaldson says the Howard government introduced the laws on enrolment deadlines to strengthen the electoral roll's integrity.

"While we welcome the additional 100,000 voters who will be participating in this election, we remain concerned this decision leaves the door open to abuse by electoral rorters," he said in a statement.

"Electoral rorters will now be able to use the busy period, after the issue of writs, to make false enrolments and false transfers of enrolments."

Senator Ronaldson said if the Coalition won government it would look into legislating "to ensure the integrity of the electoral roll".