The failure of Australia's conservative government to win any of five by-elections held over the weekend is the latest indicator PM Malcolm Turnbull will struggle to win a general election, experts have said.

"We will look very seriously and thoughtfully and humbly at the way in which the voters have responded," Turnbull told reporters in Sydney on Sunday.

In what was coined "Super Saturday", voters in five seats went to the polls after a handful of mainly Labor opposition lawmakers were forced from office after falling foul of the country's constitution that bans politicians from elected office if they are dual citizens.

The Australian Electoral Commission (AEC) on Sunday said four seats were retained by Labor and a fifth by the small Centre Alliance party

These results will come as a blow to Mr Turnbull, who is under pressure to demonstrate a viable pathway for re-election with his coalition badly trailing Labor in opinion polls.

Australian MP Bob Katter’s bizarre response to gay marriage question

Political analysts say Mr Turnbull must win marginal seats in the state of Queensland - traditionally a conservative heartland - if he is to be re-elected.

But his government failed to win a key by-election seat in Queensland, despite support from the right-wing One Nation party.

Labor retained the Queensland seat of Longman with a bigger majority than it had secured in 2016.

Australia 'faces rebuilding of postwar proportions' Show all 11 1 /11 Australia 'faces rebuilding of postwar proportions' Australia 'faces rebuilding of postwar proportions' 534202.bin An aerial view of the Ipswich CBD submerged by floodwater GETTY IMAGES Australia 'faces rebuilding of postwar proportions' 534182.bin Houses submerged by floodwater near Ipswich, west of Brisbane, yesterday EPA Australia 'faces rebuilding of postwar proportions' 534183.bin Displaced elderly citizens arrive at the QE2 Evacuation Centre, Nathan, in Brisbane south EPA Australia 'faces rebuilding of postwar proportions' 534184.bin Evacuated flood victims at the RNA Showgrounds in Brisbane yesterday GETTY IMAGES Australia 'faces rebuilding of postwar proportions' 534185.bin A fridge floats down the street in a Brisbane suburb EPA Australia 'faces rebuilding of postwar proportions' 534186.bin A swamped motorway west of Brisbane EPA Australia 'faces rebuilding of postwar proportions' 534187.bin A soldier carries a baby to safety in Gatton, west of Brisbane REX FEATURES Australia 'faces rebuilding of postwar proportions' 534203.bin Residents use a boat to navigate across their flooded suburb as flood waters inundate Brisbane GETTY IMAGES Australia 'faces rebuilding of postwar proportions' 534181.bin A precarious journey along a street in the Brisbane suburb of Laidley, Queensland REX FEATURES Australia 'faces rebuilding of postwar proportions' 534204.bin An aerial view of the Brisbane CBD area with flooded streets GETTY IMAGES Australia 'faces rebuilding of postwar proportions' 534205.bin Suncorp Stadium, the iconic Queensland venue for rugby and football matches, is submerged GETTY IMAGES

Failure to win at least one seat at the by-elections also means Mr Turnbull will continue to struggle to pass legislation with only a one-seat majority.

Opposition Leader Bill Shorten, who was targeted by the government over his poor personal approval ratings, said the results reflected Labor's focus on core issues such as health and education.

"It renews my conviction to double down in fighting for fairness and challenging growing inequality," Mr Shorten told reporters.