Archbishop John Ribat says Attorney-General Kerenga Kua's plan to activate the never-before-used law is the wrong way to deter crime and may encourage payback as families seek revenge for an execution.
"The payback system can escalate. Killing will give rise to more killing," he said.
"If the church really is against it, the government will be really slow to promote this. The government has respect for the church."
PNG is home to almost two million Catholics, who comprise 27 per cent of the population.
The death penalty is part of PNG's criminal code but the sentence has never been carried out.
"The reason it didn't move forward to implementation is we did not have the methodology," Attorney-General Kerenga Kua told AAP.
"The mechanism has not been prescribed ... coupled with the political will not being there.
"But it is there now."
He says there are currently 10 convicted criminals who have been sentenced to death.
Mr Kua says crimes such as piracy, aggravated murder and treason should attract the sentence.
"A very narrow band of crimes allow for the death penalty but it only applies in the most aggravated circumstance of wilful murder," he said.
"Then you have piracy, which is growing in the country.
"We need to stamp it out."
National Capital District governor and human rights lawyer Powes Parkop also opposes the plan.
"It is an inhuman form of punishment and therefore should be unconstitutional," he told The National newspaper last week.
"There is no empirical evidence to show that such punishment has been a deterrence to crime generally or the type of crime for which it is prescribed as a penalty."
He says in the United States, where some states have the death penalty, there is no evidence to show it has reduced crime.
The last death sentence carried out in PNG - by hanging - was during Australian colonial rule in 1954.