"My party supports the immediate legalisation of all drugs that are considered safer than alcohol and the gradual legalisation of all other recreational substances," Mr McGrath said.
"There is no longer a place for the enforcement of puritanical laws that make people's bodies the property of the state," he said.
Mr McGrath said he has worked as a doctor in the field of alcohol and drug dependence and believed that "drug use is a health issue, not a legal one".
Legalisation of drug possession in Portugal over the last 10 years had resulted in less drug use overall, including in the under 18 age group, lower rates of HIV infection, and more people coming forward for assistance with problems associated with drug use, he said.
There was no reason to be frightened of giving people back sovereignty over their bodies, he said.
"Fundamentally, the issue of drug use is a moral one, with the fundamental question being: Who owns your body - you, or the politicians?" Mr McGrath said.
"The Libertarianz Party supports Don Brash on this issue, and believe what applies to cannabis should equally apply to other substances as well."
Mr Brash told the New Zealand Herald he would like the Act Party to adopt his stance as policy, but did not expect success given that the board had set up a law and order sub-committee, chaired by John Banks, Act Party Epsom hopeful and a former police minister.
Mr Banks on Tuesday dramatically shot down his party leader's views on decriminalising cannabis, saying the stance will never become party policy.
"Just because it's the view of the leader doesn't mean it has a paradigm effect on my free thinking around drugs," says Mr Banks, who is vehemently opposed to decriminalising cannabis or any softening towards drugs and alcohol.
