A Labor frontbencher says Opposition Leader Bill Shorten is secure in his position despite new polling showing Labor behind in two key super Saturday byelection races.

Labor would lose the Queensland seat of Longman and the northern Tasmanian seat of Braddon if new ReachTel polling results are replicated when voters go to the polls on July 28.

Sky News reported on Sunday the Coalition had a 52 per cent to 48 per cent lead over Labor in Longman and a 54 to 46 per cent lead in Braddon.

Labor's Susan Lamb and Justine Keay resigned in the seats because of dual citizenship.

No government has won a seat from the opposition in a byelection in Australia since 1911, seeing the polling results renew questions about Bill Shorten's performance.

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Workplace spokesman Brendan O'Connor told ABC TV's Insiders Mr Shorten was secure despite the polling, predicting the long campaigns would be bad for Prime Minister Malcolm Turnbull.

"It's a very long campaign, Malcolm Turnbull has ensured that. He likes long campaigns," he said.

"We had an eight week election campaign which of course he nearly lost, what should have been an unlosable election for Turnbull.

"Labor will put its front foot forward. We'll argue that we are there for education and health, and providing tax relief for 10 million workers."

Asked if the government snaring one of the seats from Labor would lead to a move against Mr Shorten, Mr O'Connor said the party caucus was behind the leader.

"Well, I think that firstly, Bill is absolutely secure in his position, he is doing a great job," Mr O'Connor said.

"He brought us very close to an election win when everyone had written us off at the last election, almost in one term, and we are a united, focused opposition, putting forward plans for Australia's future.

"I think our policies, as presented, make us very competitive at the next general election. We take nothing for granted but Bill Shorten is our leader and doing a very good job."

Workplace Minister Craig Laundy said the government was hopeful of winning at least one of the seats, but it would be a hard fought campaign.

"We have to be extremely cautious. No government, I think, since 1911 has won a by-election for a seat it did not hold," he said.

Mr Turnbull campaigned with Liberal candidate for the South Australian seat of Mayo, Georgina Downer, on Friday.

Voters in that seat and in Fremantle and Perth in Western Australia will elect new MPs on the same day.

Mr Turnbull said Ms Downer, the daughter of former Howard government minister Alexander Downer, had strong roots in the electorate and surrounding community.

"Georgina is presenting herself as an advocate, as a powerful advocate but one who can work within the government," he said.

The new ReachTel poll showed the Coalition still trailed Labor nationally, behind 48 per cent to 52 per cent on a two-party preferred basis.