In a statement late on Saturday, the 29-year-old announced he would step down as temporary speaker of the Legislative Assembly and deputy chair of the State and Regional Development Committee.

The Nationals MP expressed deep regret at his "mistake", saying: "Although I thought I was OK to drive, I wasn't.

"I drank and drove - I was a bloody idiot.

"I would like to sincerely apologise to the community, my friends and family for my actions."

Deputy premier and NSW Nationals leader Andrew Stoner said Mr Marshall's action were not befitting of a government MP.

"Earlier today I was informed that the MP for  the Northern Tablelands, Adam Marshall, was last night caught driving under the influence of alcohol," Mr Stoner said.

"I have since expressed to Adam my disappointment with his actions and reiterated the high standards that I expect of all our MPs."

The Armidale-based MP and former Gunnedah mayor was elected to the lower house in 2013, becoming the youngest member of the NSW parliament.

"This mistake was of my own doing and if any good comes from it, I hope it is a clear message to others, if you drink and drive you will get caught," Mr Marshall said.

"This is now a legal matter before the court and I am prepared to accept the consequences of my actions.

"I will continue to work as hard as ever for the people of Northern Tablelands and carry on my work advancing our issues and needs."