Federal Labor would have lost two of its seats if the by-elections caused by the citizenship debacle had been held in those seats this weekend, a new poll shows.
However, a senior Labor frontbencher insists Bill Shorten's position is safe as opposition leader whatever the outcome.
The Sky/Reachtel poll released on Sunday shows the coalition has a 52 per cent to 48 per cent lead over Labor in Longman in Queensland on a two party-preferred basis.
In the northern Tasmanian seat of Braddon, the poll shows the coalition has an even greater margin of 54 to 46 per cent.
Government minister Craig Laundy is hopeful of a win but warned it wouldn't come easy.
"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 told Sky News on Sunday.
The by-elections will be held on July 28, alongside those in Mayo in South Australia and Western Australia's Fremantle, which were also caused by dual-citizenship problems for sitting members.
A fifth by-election in Perth was caused by the sitting member Tim Hammond resigning for family reasons.
Labor's Brendan O'Connor believes his party will do very well in what is a long campaign despite these early poll results.
"But it is a contest. These are marginal seats in anyone's language, they are not easy," he told ABC television on Sunday.
Asked if his party's leadership would become an issue should Labor lose a by-election, Mr O'Connor said: "I can assure you that the caucus is fully behind Bill Shorten as leader."
"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."
Despite the predicted success for the Turnbull government in two Labor seats, on a nationwide basis, it still lags Labor 48 per cent to 52 per cent.
Both major parties lost ground in the primary vote with the coalition down one point to 35 per cent and Labor down one point to 34 per cent.
The Greens picked up one point to 11 per cent, while One Nation jumped three points to nine per cent despite recent internal problems between leader Pauline Hanson and one of her senators Brian Burston.