Legalising marijuana for recreational use would boost the budget by up to $1.8 billion a year, the Parliamentary Budget Office has revealed.

The independent costing of the policy submitted by the Greens shows a tobacco-style 25 per cent excise on each sale with a 10 per cent Goods and Services Tax and a reduction in law enforcement would net $3.5 billion by 2020-21.

Marijuana tourists travelling to Australia would add up to "10 per cent of total sales", earning $130 million in revenue by 2020.

Greens leader Richard Di Natale said the boost would be used to fund drug education and treatment programs.

“The Greens see drug use as a health issue, not a criminal issue," he said.