Australia’s ruling rightwing coalition has been pushed to the brink in a shock election result that saw prime minister Malcolm Turnbull’s Liberal party lose at least 11 seats in the 150-strong House of Representatives.

At a speech given after midnight in Sydney, Turnbull claimed the Liberal and National parties would form a coalition majority government, even though he had been advised that the result would not become certain until at least Tuesday.

On Saturday 30% of the vote was still to be counted. “Based on the advice I have from party officials, we can have every confidence that we will form a coalition majority government in the next parliament,” Turnbull said. The coalition had 68 seats to Labor’s 64 – eight short of a majority and well short of the 90 that it won when it took office from Kevin Rudd’s divided Labor government in 2013.

Turnbull accused Labor of running “some of the most systematic, well-funded lies ever peddled in Australia” in a campaign in which Labor claimed the coalition was planning to privatise the government-funded Medicare system. He questioned whether there would be a police investigation over the Labor campaign and accused Labor of sending texts to voters claiming the coalition would sell Medicare.

The result places Turnbull under extraordinary pressure from the conservative wing of his party, who were criticising some of Turnbull’s key policies, including an end to superannuation tax breaks for the rich.

In an upbeat speech in Melbourne, Bill Shorten, the Labor leader, claimed that the Liberal party had lost its mandate.

“We will not know the outcome of this election tonight,” Shorten said. “Indeed, we may not know it for some days to come. But there is one thing for sure – the Labor party is back.

“Three years after the Liberals came to power in a landslide, they have lost their mandate.”

Shorten said the result was a clear rejection of Turnbull’s “ideological agenda” and Turnbull could no longer claim he could deliver stability.

Senior Liberals were fuming at Labor’s Medicare campaign in the final two weeks of the campaign, with treasurer Scott Morrison, deputy Liberal leader Julie Bishop and employment minister Eric Abetz all blaming the Labor campaign for the seat loss.

But they were also forced to defend the Liberal party campaign and the leadership change from Tony Abbott to Turnbull last September.

Asked whether the party’s leadership change had made a difference to the election result, Morrison said: “Look, that is a matter we will never know … I think it’s highly unlikely,” Morrison went on. “I think the party room made its own judgment last September.”

Bishop defended the Liberal party’s strategy as a positive campaign “of integrity”.

Asked about the effect of the change of leadership, Abetz, one of Abbott’s key supporters, said: “Suffice to say a change was made for better or for worse. We move on and we’ve got to ensure Malcolm Turnbull is returned as prime minister for the sake of the nation.”

But Abetz committed to take the coalition’s superannuation plan to a meeting of the party after the election, blaming the policy for a “haemorrhage” of votes. “The emails coming into my office were very strident in their criticism: there was the view there was retrospectivity; there was the view they had worked hard, saved hard, and door-knocking was also a recurring theme,” Abetz said.

“Regrettably some wanted to punish us for that, and we did fight very hard, saying be careful you don’t jump out of the frying pan into the fire. I fear some of them may have done that and regrettably we did haemorrhage some votes.”

Deputy prime minister Barnaby Joyce held off a challenge from the former independent Tony Windsor.

The Australian Electoral Commission estimated that 10 million Australians voted. Electoral commissioner Tom Rogers apologised for unusually long waits at polling booths.

Rogers said changes to the senate voting rules were “the most significant changes to voting in 30 years. This, in combination with record nominations in some seats, appears to have resulted in voters taking greater care and more time to cast their vote.”