A new law giving security guards in detention centres power to cause grievous bodily harm if they "reasonably believe" it is necessary to protect life or prevent injury is likely to encourage abuse of and violence against asylum seekers in detention, a Senate committee has been told.

Former judge of the Victorian Court of Appeal Stephen Charles QC said the law allowed security guards to use lethal force "with impunity" because it would be "almost impossible" for them to face prosecution in the courts.

The Maribyrnong immigration detention centre in Melbourne was the harshest facility in Australia in 2014-15, according to previously obtained data. Credit:Shannon Morris

"These amendments will authorise detention centre guards to beat asylum seekers to death if they reasonably believe it is necessary to do so to save either themselves or another person from serious harm," he said.

Mr Charles cited a legal opinion that the police officer who shot a black American eight times in the back was likely to escape a murder conviction because of the way the "reasonable belief" test has been applied in the United States.