CANBERRA, July 28 (Xinhua) -- The Australian government is unlikely to win any additional seats in Saturday's by-elections, leaving it with a tenuous grip on power.

Hundreds of thousands of Australians have headed to the polls to vote in five by-elections on what has been dubbed "Super Saturday."

According to the latest Newspoll, released on Saturday, the opposition Australian Labor Party (ALP) is expected to retain four of the seats, including Longman in Queensland and Braddon and Tasmania - two seats where the governing Liberal National Party (LNP) posed a significant threat.

The poll of more than 2,000 voters in the two marginal electorates found that 51 percent of voters in both seats preferred the ALP over the LNP on a two-party preferred basis.

The news was worse for the LNP in the South Australian seat of Mayo where the Centre Alliance, a minor party which currently holds the seat, led 59-41 over the government.

Retaining all four seats currently held by the party would be a significant boost for ALP leader Bill Shorten with many referring to the by-elections as a referendum on the leadership of Shorten and Prime Minister Malcolm Turnbull.

Failing to win a single seat would leave the LNP with a narrow one-seat majority in parliament and only barely hanging on to power.

If the LNP is able to cause an upset it would be the first time an Australian government has won a seat off the opposition at a by-election in 98 years.

Turnbull's biggest hope is in Longman where preferences from the right-wing One Nation Party, which is expected to garner 14 percent of first preferences, could see the government overcome the two-party preferred deficit.

Four of the five by-elections were triggered when the sitting Members of Parliament (MPs) in those seats were forced to resign after it was found that their dual citizenship rendered them ineligible to serve in parliament.