In March 1865 Macadam sailed to New Zealand to give evidence at the trial of Captain W. A. Jarvey, accused of fatally poisoning his wife, but the jury did not reach a verdict. During the return voyage, Macadam fractured his ribs during a storm. He was advised, on medical grounds, not to return for the adjourned trial but did so and died on the ship on 2 September 1865. His medical-student assistant John Drummond Kirkland gave evidence at the trial in Macadam's place, and Jarvey was convicted.
The Australian News commented, "At the time of his death, Dr Macadam was but 38 years of age; there can be little doubt that the various and onerous duties he discharged for the public must be attributed in great measure the shortening of his days." The Australian Medical Journal stated, "For some time it had been evident to his friends that his general health was giving way: that a frame naturally robust and vigorous was gradually becoming undermined by the incessant and harassing duties of the multifarious offices he filled." The inquest verdict (he died at sea) stated, "His death was caused by excessive debility and general exhaustion."