PNG Prime Minister Peter O'Neill during the week said his government planned to apply the death penalty, which has not been used in PNG since 1954, to convicted murderers.
"I discussed the death penalty. The prime minister spoke about the challenge of law and order in Papua New Guinea," Senator Carr reporters on Saturday.
"I said to him, I said to foreign minister (Rimbink) Pato, Australia is opposed to the death penalty in all circumstances. We never cease to make that clear."
Mr O'Neill said this week his government had discussed using firing squads as a method of execution.
The proposal - due to be debated in PNG's single house of parliament in mid-May - is part of a raft of legislation aimed at toughening sentences for alcohol, drug and rape offences.Senator Carr arrived in Port Moresby on Friday and gave a speech to the Burnet Institute, a not-for-profit medical research organisation.
His visit comes ahead of the expected arrival of Prime Minister Julia Gillard to PNG in the coming week.