A row has erupted within the WA Labor Party with a union official beaten in a ballot for the role of president planning to dispute the outcome.

Maritime Union of Australia candidate Adrian Evans was beaten to the position by left-wing unionist Carolyn Smith this week after a vote closed on Tuesday.

He has now written an email — obtained by the ABC — to Labor members, saying he has informed WA ALP state secretary Patrick Gorman "he should expect a notice of dispute in coming days".

"I rang Carolyn Smith this morning to congratulate her but also to let her know that I intend to lodge a dispute which could potentially overturn the outcome or cause a re-run," Mr Evans wrote.

The twist comes after Premier Mark McGowan raised eyebrows in April when he publicly backed Ms Smith, the secretary of Labor's most powerful union United Voice, to replace Peter Tinley as president for the next two years.

"While it wasn't a victory just yet, 48 per cent in a state-wide member ballot against the Premier's pick [is] a significant level of support and shows our push to engage rank-and-file members is well supported," Mr Evans wrote.

"Carolyn pulled in a lot of endorsement from politicians, our campaign was rank-and-file driven. I am extremely proud of that."

Mr McGowan does not normally involve himself in internal party matters.

His glowing praise of Ms Smith ahead of the ballot prompted some senior Labor figures to claim it was related to the Premier being indebted to United Voice for help it provided in seeing off last year's challenge for the Labor leadership from former federal defence minister Stephen Smith.

Concerns over ballot papers

Ms Smith beat Mr Evans by 141 votes and was declared the winner on Tuesday night after more than 3,000 rank-and-file postal votes were distributed as part of the ballot.

Carolyn Smith won the ballot to become president after being backed by Mark McGowan. ( Supplied: Facebook )

Mr Evans' email appears to indicate he plans to dispute the ballot outcome because of issues with his union members not getting postal ballot papers.

"We had plenty of issues with our members getting a vote that we needed to fix before the ballot closed, but I am sure there are more members that did not get a resolution before then," Mr Evans said in the email.

"If you were not able to vote due to issues with the party, please let me know ASAP."

The ballot for president was widely seen as the first real test of which factional grouping has control of the ALP, after a number of unions realigned earlier this year to form a new grouping know as Progressive Labor.

Progressive Labor comprises a number of unions previously in opposing factions, and includes the Maritime Union of Australia, the CFMEU, the Transport Workers Union, the Shop, Distributive and Allied Employees Association and the Australian Workers Union.

They have joined forces in an attempt to limit the influence of Ms Smith over party preselections and key appointments to the ALP's decision-making bodies.