A volunteer firefighter has pleaded guilty to three counts of deliberately lighting a fire likely to cause injury or damage in Western Australia's South West.

Stephen Kenneth Johnson, 23, was arrested after a joint operation conducted with South West Detectives into deliberately lit fires within the Donnybrook area that damaged property and bushland in December 2015.

Stephen Kenneth Johnson, 23, admitted starting the fires that threatened homes around Donnybrook and Irishtown. Credit:Department of Fire and Emergency Services

Johnston, who was a volunteer firefighters with the Argyle Bushfire Brigade and a member of the Donnybrook SES, admitted starting the blazes that threatened homes around Donnybrook and Irishtown

Two of the fires are alleged to have occurred on December 22 with a third fire happening on December 28. The third fire was estimated to have had a significant financial impact on community and emergency services.

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Johnson will be sentenced in Bunbury District Court on June 20.

The maximum penalty for these offences is 20 years imprisonment.