A South Australian Labor MP has been arrested and charged with child pornography offences.

The MP cannot be named for legal reasons but police arrested him at his home on Wednesday evening and took him to the city watch house.

Investigators spent some time inspecting a car parked in the driveway at the man's house.

They used plastic gloves to search the front and back seats as well as the boot and later emerged from the house carrying brown paper bags.

The MP has been charged with possession of child pornography, aggravated possession of child pornography and two counts of taking steps to access child pornography.

Police say the man was arrested as part of Operation Decimate, which targets offenders who use the internet for child exploitation-related offences.

He has been released on bail and will appear in the Adelaide Magistrates Court on May 20.

Media advisers for both South Australian Premier Mike Rann and Liberal Opposition Leader Isobel Redmond would not comment when contacted by ABC Radio's AM.

Mr Rann has flown back into Adelaide from interstate this morning and issued a statement.

The Premier says he is shocked and has urged anyone with any evidence or information about the allegations to cooperate fully with the police.

A visitor to the MP's home this morning described the matter as an "absolute nightmare" and said "there has to be some kind of mistake".

Arriving for an Executive Council meeting this morning, some MPs refused to comment and others expressed shock.