Polling booths are open across South Australia as a record number of voters decide the next government on Saturday in what many see as the most unpredictable election in the state’s recent history.

Labor is trying to secure a fifth consecutive term in office, after 16 years in power, as it is challenged by Steven Marshall’s Liberals and Nick Xenophon’s new party SA Best.

Support continues to slip for SA Best, according to The Australian’s Newspoll, with the fledgling political party on 17 per cent, compared with the 32 per cent support it recorded late last year.

The poll puts Jay Weatherill ahead of Mr Marshall as preferred premier, with Mr Xenophon now trailing both as he tries to return to the SA parliament.

Electoral boundaries have changed markedly since South Australians last went to the polls four years ago, when the Liberals won the popular vote but failed to form government.

A record 1.2 million voters are registered for the 2018 election.

Labor has campaigned on planned capital works of nearly $2 billion, the Liberals have committed to tax cuts and Mr Xenophon has not revealed costings, but urged voters to look at his track record at state and federal levels over two decades.

The Australian Electoral Commission said a record of more than 1.2 million South Australians are registered to vote – an increase of more than 60,000 since 2014.

Many people have already cast their ballots.

More than 215,000 people voted before election day itself – including Liberal leader Mr Marshall – 120,000 at pre-polling centres and about 95,000 by post.

The 2014 state election produced a hung parliament, and there are many people who think today’s poll might do the same.