The prime minister, Malcolm Turnbull, insists his Senate woes would have been worse if he had not called a double-dissolution election.

Addressing the Liberal party faithful at the Tasmanian state council meeting on Saturday, Turnbull said predictions that the Senate with its 11-strong crossbench could not work were based on “mythology and conjecture”.

He rejected accusations that it was his dissolution of both houses of parliament, triggering an election of all Senate seats rather than only half as is typically the case, that led to an unwieldy upper house.

“If we had had a half Senate election, we could have very reasonably expected to have the same number of crossbenchers elected as there had [been] three years before, and then you would have 12,” Turnbull said.

The double-dissolution has been blamed for the expanded crossbench, which swelled from eight to 11 at the 2 July election, because the quota needed to get elected is lower than at a half-Senate election.

It comes as two crucial crossbench seats remain in doubt for at least several weeks –the high court is likely to be asked to determine whether the former senator Bob Day and One Nation’s Rod Culleton were illegally elected.

Turnbull said the double-dissolution and his government’s changes to the way Australians elected senators had paved the way to ensure legislation to re-establish the building industry watchdog would pass.

“It had absolutely no prospect of passage in the last Senate,” he said.

“[It] would have had no prospect of passage in the next Senate, had we gone to a half-Senate election.”

The government now has the option of a joint sitting of both houses to pass its industrial relations reforms if the new Senate rejects them.

Turnbull paid tribute to the Tasmanian MPs who lost their seats at the election, including the so-called “three amigos” Andrew Nikolic, Brett Whiteley and Eric Hutchinson.

“We must make sure that when the next election comes we remind the electors of those three seats that it was the three amigos who secured the investments,” he said.

Having suffered heavy losses at the election, the Coalition needed to learn how to counter Labor’s aggressive ground game, supported by unions and activist groups such as GetUp, Turnbull said.

But the Liberal party also needed a much larger and more representative membership, he said. The capacity of members to contribute to policy also needed to be stronger.

“We must also look within and transform our party, so that we can face future elections in a better position to win back those seats.

“We need a bigger, broader grassroots base. It is critical for us to do that.”

One resolution on the state council agenda calls for the Australian Electoral Commission to change the colour of the House of Representatives ballot paper from green because it provides “free subliminal advertising for the Greens inside polling booths”.

The paper is green because that is the colour of the lower house chamber in Parliament House, which mirrors the House of Commons in the UK.

Another motion calls for an end to the five cent coin, which the Southern Young Liberals say is becoming irrelevant.

There will also be debate on a motion to abolish the Safe Schools anti-bullying program, which has been criticised by conservatives within the party because of its emphasis on gender and sexual diversity.