Religious oaths in courts could be replaced by a secular promise under a proposal from the South Australian Law Reform Institute.

Replacing the oath with an affirmation, which is currently optional, will put all witnesses on a level playing field and mean more to non-religious witnesses, director John Williams says.

The SA Law Reform Institute will present its report into courtroom and tribunal practices to Attorney-General John Rau on Tuesday.

"There is a belief (with jurors) that if somebody swears on the Bible, they're telling the truth more than someone who is making an affirmation," Professor Williams told AAP.

"Some witnesses have been known to swear on a Bible because that's what you do to be seen as more truthful, so it's open to manipulation."

But Prof Williams said adopting a uniform affirmation would not stop people lying in courts.

"If someone is going to lie under oath, they can lie under an affirmation. But there will be no room for people to say `I didn't understand what you were asking me'."