Deposition proceedings against the gang began on 2 August 1866 amid great excitement. Only now was it revealed that Sullivan had informed on the others. On 7 August Burgess wrote a long statement - 'The Confessions of Burgess the Murderer' - in which he detailed his many crimes and exonerated Kelly and Levy.
The case went to the Supreme Court. A special sitting opened in Nelson on 12 September with Mr Justice Johnston of Wellington as trial judge.
In accordance with the terms of the amnesty, Sullivan was not charged. Burgess, conducting his own defence, was determined to implicate Sullivan directly in the killings and cross-examined him for 15 hours without success. When Burgess asked him why he had killed James Battle, the judge advised Sullivan that he did not have to answer the question.
On 18 September Johnston spent more than six hours summing up the case for the jury. He described Burgess as an 'arch plotter' , a 'cruel assassin' and 'one of the wickedest of men'.
The jury took less than an hour to find all three men guilty of murder. Kelly collapsed and was taken away sobbing, while Levy continued to maintain his innocence. Convictions for the killing of Mathieu's party had been secured by the decision of those involved to accept Sullivan’s testimony, whatever private doubts they may have held about its reliability.
Sullivan escapes death
The amnesty applied only to the murder of Mathieu and his associates. When a separate trial for the murder of James Battle was held on 19 September, the jury took just 25 minutes to find Joseph Sullivan guilty. He was sentenced to death.
Regardless of how helpful Sullivan had been to the police, the public believed he was a murderer who had got what he deserved. Sullivan claimed he had a deal with the government, and the police supported his plea for a pardon. Police informants were key crime-solvers, and future investigations might suffer if potential grasses doubted the sincerity of deals offered for information. The Superintendent of Nelson Province, Alfred Saunders, agreed. Sullivan's sentence was commuted to life imprisonment, and he was formally pardoned in 1874.