The coalition's grasp on the West Australian federal seat of Canning appears to have slipped further just a month out from a by-election.

Liberal MP Don Randall, who died of a suspected heart attack last month, had a lead of almost 12 per cent, but according to the latest Newspoll that margin has now all but disappeared.

The poll, which surveyed 508 Canning voters last weekend and was published by The Australian, shows the government's 61.8 per cent two-party-preferred lead over Labor's 38.2 per cent at the last election has fallen away to a gap of just 51 to 49 per cent.

Political analyst David Black said he expected Canning would remain a Liberal seat, but he assumed there would be a significant swing towards Labor.

"The polls suggest that the government is not travelling all that well and so I think the assumption is it would be very surprising if they can get anywhere near the same result as last time," Prof Black told AAP.

"There was a degree of popular support for Don Randall.

"So my assumption at the present time is the chances of the seat being lost are probably very low.

"That being said, the extent of the swing will be of considerable interest."

If Labor did win, that would be disastrous for the coalition given the by-election was being seen as its popularity test, the Curtin University professor said.

Labor's candidate would do well if they matched the performance of Mr Randall's sparring partner Alannah MacTiernan, who unsuccessfully contested the seat in 2010, leaving him with a two per cent margin.

He bumped it up to double figures in 2013 in the wake of Labor's leadership crisis.

Prof Black said the military background of the likely Liberal candidate, Afghanistan war veteran Andrew Hastie, was an advantage given that showed he was a responsible person and had served his country.

But the Labor favourite, Law Society of WA president and policy adviser in the Gallop government Matthew Keogh, was prominent.

"I'm assuming it will be an interesting contest," Prof Black said.

If Mr Keogh lost, he could be a "walk up start" for a new Baldivis-based federal electoral district proposed under a redistribution of Western Australia's boundaries.

Opposition Leader Bill Shorten told the Labor caucus last week that by-elections in seats in which an MP had died tended to swing away from the former member's party by only about 2.5 per cent, but that the party would still take up the fight in Canning.

The by-election will be held on September 19.