Federal Labor risks losing the key Queensland seat of Longman in the by-election being recontested by Susan Lamb.
Ms Lamb is trying to return to the lower house after being forced to resign because she held dual citizenship, in breach of the constitution.
Labor is sitting on 49 per cent against 51 per cent for the Liberal National Party's Trevor Ruthenberg on a two-party preferred basis, according to a ReachTel poll conducted for The Courier Mail.
A week ago, the two major parties were on 50 per cent each.
The poll also noted a rise in the primary vote for minor parties such as One Nation, whose candidate Matthew Stephen was on 14.7 per cent of the primary vote.
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The poll also found that a majority - 71 per cent - of voters leaning toward minor parties said they would most likely preference the LNP over Labor, The Courier Mail reported.
The by-election is one of five being held around Australia on July 28.
