The full bench of the High Court ruled parts of the Electoral Act were unconstitutional on Friday, upholding a challenge to the electoral law changes made by the previous Howard government in 2006.
Greens leader Bob Brown says the laws should not have been passed in the first place.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"The Howard government's decision to cut voters out of their rightful ability to enrol to vote in the week after the election's called was wrong," he said.
"The Rudd government moved to change that law, and the Greens supported it, but (Family First senator) Steve Fielding was opposed to it.
"This is why it's so important for the Greens to hold the balance of power."
Sen Brown said the laws were politically targeted by the coalition.
"This was John Howard trying to cut young Australians out of voting because the coalition's not doing so well there," he said.
"In fact, recent polls show as many young people vote Green as for the coalition."
Sen Brown said people should be allowed to enrol at a much later point in the election campaign.
"We've now got the technical wherewithal to allow people, if they're qualified, to enrol to vote right up to election day," he said.
"Other countries do it, we should do it too."
