The safe West Australian Liberal seat of Canning may have become a marginal seat on the eve of a crucial by-election, a new survey has suggested.

The union-commissioned ReachTEL poll put Labor’s candidate, lawyer Matt Keogh, slightly ahead on the two-party preferred basis at 50.1 per cent, with the Liberals Andrew Hastie, a former SAS captain, on 49.9 per cent.

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It is a slump on 2013 election figures that saw former Liberal representative Don Randall win the seat with 61.8 per cent of the two-party preferred vote.

Mr Randall died on July 21, instigating the by-election process.

A loss in the Perth seat could spell trouble for the embattled Coalition Government when it may face its first real electoral health check.