A special breakdown of voting figures from the May 18 poll suggests less than 91 per cent of people cast a ballot, formal or informal.

It is on track to be lower than the 2016 election and the worst result since the mid-1920s (excluding some years when up to a dozen seats had just one candidate) when compulsory voting was introduced after just 55 per cent of Australians voted at the 1922 general election.

Nearly 16.5 million - 96.8 per cent of eligible voters - were enrolled to vote, the most complete the electoral roll has been since Federation. This was in part due to an influx of younger voters who came onto the roll for the same-sex marriage postal vote in 2017.

But up to 1.5 million people on the roll failed to vote at the election. In some seats, once informal votes are taken into account, less than three-quarters of those entitled to vote cast a legitimate ballot.

The biggest falls were in a string of inner-city electorates which have high proportions of young voters.

In Melbourne, the youngest seat in the country with a median ago of 30, turnout is below 82 per cent. At the 2013 election, 90 per cent of the electorate's voters cast a ballot.

In Sydney, held by former Labor deputy leader Tanya Plibersek, the median age of voters is 32. There the voter turnout fell to a similarly low level after being as high as 90.8 per cent at the 2007 election.

Brisbane, Perth and Adelaide - all either the youngest or among the youngest electorates in their respective states - had among the lowest state turnouts.

Some close seats with relatively large populations of young people, such as Swan in Western Australia and Chisholm in Victoria, could have changed hands if turnout had been higher.

Outside the capital city seats, every electorate with a median age of 33 or less - bar Holt in Victoria - had a turnout rate less than 90 per cent.

By contrast, across the five electorates of Tasmania, the nation's oldest state with median ages of up to 45, voter turnout slightly increased with each one above 92 per cent. The NSW electorate of Lyne, the oldest in the country with a median age of 50, also had a similarly high turnout.

But where there was a real contest, voters responded by turning up.

The battle between Tony Abbott and independent Zali Steggall in the seat of Warringah resulted in the best turnout in that electorate for at least two elections.

In the Victorian seat of Flinders, where Health Minister Greg Hunt faced a battle from former Liberal Julia Banks plus the Labor Party, the turnout also bucked the recent trend with an increase in the proportion of voters turning out.

Not one federal electorate has a turnout matching what they recorded at the 2007 poll. If that turnout rate of 94.8 per cent had been replicated on May 18, up to 600,000 more votes would have been cast.

Lead researcher of the Australian National University's Australian Election Study, Ian McAllister, said it appeared younger people were increasingly isolated from older Australians when it came to democracy.

He said the same-sex marriage plebiscite had put young people on the roll but they had failed to find a reason to vote in the general election.

"They're not socialised into the whole experience of voting," he said.

"They're busy with their lives and even though they've enrolled to vote they're just not that interested in going through with the process."

NSW independent MP Alex Greenwich, former co-chair of the 'yes' campaign, blames the major parties for not including younger voters in the national discussion, even when discussing climate change.

"My big concern was the messaging from the major parties - a lot of the debate was about retirement, and even when we were discussing things like climate change we were having an ideological debate rather than an action-based one," he said.

Apart from falling turnout, the number of informal votes nationally rose at the election.

Blaxland, held by Labor's Jason Clare, recorded a nation-high informal rate of 13.4 per cent. On top of its low turnout rate of under 85 per cent, barely 70 per cent of the seat's voters cast a valid ballot at the election.

One Liberal MP said the voting figures suggested ramifications for the political system and major parties.

"Everyone campaigns on the assumption that people vote. This might mean they will have to campaign on the assumption they have to get people to vote," they said.

The current penalty for not providing a "sufficient reason" for voting is $20.