Malcolm Turnbull has been dealt a blow before the mammoth nine-week Super Saturday byelection campaign begins as the government drops further behind Labor.

The latest Newspoll, published on Sunday night by The Australian, found the Liberal-National’s two-party preferred vote had fallen one point to 48, to Labor’s improved 52. The government’s primary vote also dropped one point to 38, equal with Labor.

It is the 33rd Newspoll in which the government has trailed Labor.

In some good news for Prime Minister Malcolm Turnbull, his lead over Labor leader Bill Shorten extended by one point to a margin of 17 percentage points. This means Mr Turnbull is the preferred PM at 47 per cent to Mr Shorten’s 30 per cent.

The poll comes days after Speaker Tony Smith controversially announced the byelections, including two marginal seat contests, would be held on July 28, leaving the five seats vacant for 78 days.

The extensive delay, which Mr Smith defended as reflecting the electoral commission’s advice, infuriated Labor, which accused the government of a “sneaky tactic”. The party’s national conference had been scheduled for the same day.

The poll was conducted as the government seized on internal debate within Labor over the party’s policies on asylum seeker boat arrivals.

Frontbencher Linda Burney was forced to apologise after comments in which she erred from Labor’s policy were edited from an official transcript, while new MP Ged Kearney last week hit out at Australia’s record on refugees in her maiden speech.

After a speech to the Victorian Labor conference on Sunday, Mr Shorten told reporters that a future government he led would “stop the boats”.

“The current government would like to say that there’ll be another policy. There won’t be,” he said.

“I’m very committed to making sure the boats don’t start again.”

Cabinet Minister Greg Hunt said on Sunday that Labor was “divided” on border protection.

“We have a position which is crystal clear and that is we will not let people who come via this way to settle in Australia,” he said. 

The poll also followed a major blow for the government last week when Pauline Hanson backed out of a deal to support the Coalition’s company tax cuts.

In a further blow, Senator Hanson told 2GB on Sunday she would also not support Derryn Hinch’s compromise proposal to provide a tax cut to businesses with a turnover under $500 million, effectively carving out the big banks.

One Nation gained two points in Newspoll’s measure of primary vote, to be on eight per cent, just behind the Greens’ nine per cent.

Despite reports suggesting some Nationals MPs were lobbying the government to drop the company tax cuts, Mr Hunt said on Sunday the Coalition remained “very committed to the plan”.

Mr Shorten described the tax cuts as “rotten” during his speech to the Victorian Labor conference.

The byelections in Longman (Queensland) and Braddon (Tasmania) are expected to be the most closely contested, with the government failing to run in Perth and Fremantle.

The long campaign is also expected to favour Liberal candidate Georgina Downer, who is seeking to reclaim Mayo (South Australia) for the government from the Centre Alliance’s Rebekha Sharkie.