Surprise forecast from Australia Institute as it also predicts Centre Alliance and One Nation will hold balance in upper house

This article is more than 1 year old

This article is more than 1 year old

Centre Alliance and One Nation are likely to control the Senate balance of power regardless of whether Labor or the Coalition win Saturday’s election, according to an analysis by the Australia Institute.

The thinktank believes the best possible Senate result for the Coalition is 34 seats while Labor and the Greens could at most win 37, leaving major parties reliant on one or both of the minor parties to pass legislation in the upper house.

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But in a surprise result, the progressive thinktank predicts the United Australia Party will fail to enter the Senate, although party leader Clive Palmer is an outside chance of passing One Nation’s Malcolm Roberts to claim a seat in Queensland.

The Australia Institute analysis uses the average of the last three polls of Senate voting intention, conducted by Dynata, with an extra Lonergan poll in May conducted in Tasmania, Queensland and South Australia.

The most contentious states are South Australia and Tasmania where the Liberal party, Greens and One Nation will compete for the last two seats, with Centre Alliance also a contender in SA and Jacqui Lambie in Tasmania.

Facebook Twitter Pinterest One Nation leader Pauline Hanson could control up to four seats in the upper house after Saturday’s election. Photograph: Mick Tsikas/AAP

With a vote of 10.7% in Queensland, Roberts is on track to win the last Senate seat, joining Pauline Hanson in the Senate. One Nation could also win Senate seats in South Australia or Tasmania, for a total of two to four seats.

With a vote of 6.2% in South Australia, Centre Alliance could gain an extra senator to join Rex Patrick and Stirling Griff, who are not up for re-election.

That bloc of four to seven One Nation and Centre Alliance senators is predicted to hold the balance of power in a Senate where the Coalition will hold 32 to 34 seats, and Labor 28 at most.

With the Greens expected to win seats in New South Wales, Queensland, Victoria and Western Australia and three senators not up for re-election the party will have at least seven seats, or up to nine if it wins in South Australia and Tasmania.

Cory Bernardi, who quit the Liberal party after winning a six-year term, will remain on the crossbench and could be joined by Lambie in Tasmania, who is polling 8.4%.

The polls find UAP on 3.3% in Queensland, 3.6% in South Australia and 2.9% in Western Australia – not enough to suggest electing senators is likely, but a dark horse if the polls systematically understate its vote.

On Wednesday Guardian’s Essential poll found the Greens on a primary vote of 9.1%, One Nation on 6.6% and others/independents on 9.6%. The latest Newspoll has the Greens on 9%, One Nation on 4% and others on 7%.

The 2016 double-dissolution election returned an eclectic crossbench including nine Greens, four One Nation senators, three for the Nick Xenophon Team (now Centre Alliance), one Family First, one Liberal Democrat, Lambie and Derryn Hinch.

The 2019 election will be the first half-Senate election under new voting rules, which allow voters to number at least one to six above the line.

Before 2016 voters could nominate only one party above the line, with preferences distributed according to that party’s ticket lodged with the Australian Electoral Commission.

The Australia Institute analysis suggested in the best case for the Coalition on 34 seats it could pass legislation with One Nation and Centre Alliance, while in the worst case of 32 seats it could require every crossbench vote including Palmer in Queensland and Lambie in Tasmania.

In Labor’s best case scenario it (28) and the Greens (9) will be just shy of a majority, requiring two votes from One Nation, Centre Alliance or the rest of the crossbench. In the worst case the left grouping will need four votes, still leaving Labor with more options than the Coalition.

The Australia Institute’s executive director, Ben Oquist, said the crossbench “will remain diverse and powerful in the 46th parliament”.

“South Australia and Tasmania could be the crucial states in determining the final make-up and dynamic of the Senate,” he said.