The validity of a union vote to call off an impending Sydney train strike is being questioned after up to a third of workers said they hadn't received an automated text ballot.

Workers must reply "yes" to a text message from the NSW Rail, Tram and Bus Union to support calling off a proposed 24-hour strike on Monday, with a non-reply counted as a "no" vote.

Sydney commuters have been warned to brace themselves for major disruptions if the strike goes ahead. Credit:Ryan Stuart

However, many workers posted on the union's Facebook page on Wednesday morning saying they were still to receive the text, hours before the midday deadline.

"I would like to know why this has been sent out as an SMS. There are many of us on leave at the moment that do not have our work phones with us. Also how are we going to be able to vote if we are on leave," one said.