This is a partial defence to murder. If used successfully, the survivor of a suicide pact will be guilty of manslaughter rather than murder.
s. 6B Crimes Act 1958
Survivor of suicide pact who kills deceased party is guilty of manslaughter
(1) Where upon the trial of a person for the murder of another person the jury are satisfied that the accused caused or was a party to causing the death of that other person by a wilful act or omission but are satisfied on the balance of probabilities that the act was done or the omission made in pursuance of a suicide pact then the jury shall, notwithstanding that the circumstances were such that but for the provisions of this section they might have returned a verdict of murder, return a verdict of manslaughter in lieu thereof.
(1A) Despite section 5, a person convicted of manslaughter under subsection
(1) is only liable to level 5 imprisonment (10 years maximum).