Train services will be disrupted in Adelaide on Thursday as workers strike over a pay dispute with employer Keolis Downer.

Department of Transport spokesperson Matt Bowman said services will be affected from 9am until about 3pm.

He said all train lines will be impacted, with a “small number” of substitute buses to run every 30 to 60 minutes.

“We’re doing everything we can to limit the impact to our train passengers, however the timing of this action means we’re quite limited with the number of spare buses and drivers we have available,” he said.

Mr Bowman said tram and bus services would not be impacted, and apologised for the disruption.

Commuters are warned to find alternative transport due to the strikes.(ABC News)

“Certainly we’re encouraging all of our customers who might be impacted by this to plan ahead and consider any alternative arrangements that may be available to them,” he said.

The action was sparked by a pay dispute with private contractor Keolis Downer, which has rejected workers’ demands for a 20 per cent pay increase over four years.

Union organiser Hayden Boyle said the situation was “incredibly frustrating”.

“We’ve been negotiating for six months now, really haven’t made much progress,” he said.

“I hope that [Keolis Downer] are listening loud and clear today and we’ve left them tomorrow and the weekend to think about what today means, and we’re hoping that they come back to us with a reasonable offer.”

Mr Downer said more stoppages were planned for next week if the company did not come to the table.

“We’ve got rolling stoppages for next week so Tuesday morning between 6am and 8am we’ll have a complete work stoppage and Wednesday we’ll also have an afternoon stoppage between 4pm and 6pm and the hope is the escalation will get us the result we’re after.”

Keolis Downer Adelaide apologised for any inconvenience and said it would continue negotiating “in good faith” with its staff through the union.

A company’s spokesperson said it encourages the public to keep across Adelaide Metro’s website for specific service details, as well as to seek alternative transport as required

“Keolis Downer Adelaide is currently in negotiation with employee representatives on a new Enterprise Agreement for our Adelaide Metro workforce,” the spokesperson said.

“Keolis Downer respects the right of workers to take lawful industrial action in pursuit of their claims and remains committed to negotiating an agreed outcome.”

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