In the South Australian Supreme Court today, Wendy Utting and Malcolm Barry Standfield went on trial, charged with four counts of criminal defamation.

The pair were working as volunteers in 2005 in the office of former independent state MP Peter Lewis when they faxed an allegedly defamatory letter to media outlets.

Prosecutor Adam Kimber told the court that the letter contained allegations that a serving politician, a former MP and two police officers had sex with children.

He said the allegations were completely false and extremely damaging to the men's reputation and standing in the community.

"The allegations are so gross, are so defamatory, that they had the capacity to cause serious harm," Mr Kimber told the jury.

Utting and Standfield have both pleaded not guilty, with the trial to continue for about a month.

A third accused, Craig Ratcliff, has pleaded guilty to one count of criminal defamation over allegations he made against one of the politicians.