PREMIER Mike Rann says he is shocked by reports of an MP being charged with serious criminal offences involving alleged possession of child pornography.

The MP at the centre of the allegations has been identified as a Labor MP but a six-paragraph statement released by Mr Rann makes no mention of the MP's political affiliations.

"For legal reasons, no person, including me as Premier, is at liberty to reveal the identity of the person, the subject of the allegations, or to disclose any information through which others may infer the identity of the person," Mr Rann said.

"It would be against the law for me to comment on the allegations as the matter is now before the courts.

"I believe that any potential witnesses with any evidence or information about these allegations must co-operate fully with police."

Mr Rann said the Government would, of course, co-operate with the police in the investigation.

The MP was taken to the City Watchhouse on Wednesday night, charged with four child pornography offences and later released on police bail.

He will appear in Adelaide Magistrates Court on May 20.

Detectives said their investigation was continuing.

Under SA law he cannot be identified until his committal, unless he gives his consent.

He returned home earlier today but had not spoken to the waiting media outside.

Politicians from both sides say they are shocked by the allegations.

Police said the arrest was the result of the Sexual Crime Investigation Branch's ongoing "Operation Decimate", which targets people who use the internet for child exploitation-related offences.

The maximum penalty for possessing child pornography in South Australia is five years in jail for a first offence. The maximum penalty for an aggravated offence in seven years in jail.

Section 63 of the Criminal Law Consolidation Act 1935 says that an aggravated offence includes an offender knowing that the victim was aged under 14 at the time, or the offence being committed in the company of one or more other people.

It is believed several laptops were seized from the MP's suburban home.

Turmoil swept state political circles as details of the incident emerged.

There were reports that police visited the MP's office at Parliament House but this was denied by the presiding officers, who have to give permission for police to enter the building.

Legislative Council President Bob Sneath said he had not been approached by police and Speaker Lyn Breuer said she had not been contacted by police.

Ms Breuer said she was "certainly concerned about the issue".

Labor and Liberal MPs were in shock last night and few were prepared to talk about the incident. Many were asking the media for details.

A spokeswoman for Opposition Leader Isobel Redmond said she could not comment.

It is understood the MP's home was raided on Tuesday afternoon and police spent 24 hours analysing the contents of a computer.

Yesterday, they went to the MP's office and seized another computer for analysis. Last night, police returned to the MP's house.

One officer emerged from the house about 7.35pm and went to a police car. He then carried what appeared to be a tape back into the house. At 7.55pm, two detectives wearing gloves searched a car parked in the driveway.

They examined the boot and under the seats before returning to the house. Police left the house at 8.10pm carrying several evidence bags containing compact discs and a mobile phone and the house was shut and left in darkness.

Originally published as Rann 'shocked' by child porn arrest