Adults should pay only a flat $1 fare for public transport in Queensland and people aged under 18 should travel free, the Greens say, under their state election transport policy.

Queensland Greens leader and candidate for South Brisbane, Amy MacMahon, said public transport usage had been falling over the past decade and a fare reduction would help stem the slide.

"We're anticipating that in conjunction with a review of the bus network, and making sure we have reliable services, that we'd see about a 15 per cent increase in public transport use," Ms MacMahon said.

"One dollar fares would make sure that not only would there be a cost saving for everyone, but that it's a fair and equitable price that would mean that everyone would be able to access public transport."

The party also wants to establish a consolidated public transport authority that would take over every form of public transport across the state, including the bus networks that are currently run by councils in different towns and cities.

"I think people don't really care who's looking after the public transport system, people just want to make sure there's someone in charge to make sure everything is running on time, to make sure their services are reliable, and affordable and accessible for everyone," she said.

"At the moment public transport is too slow, it's too unreliable and it's forcing people to drive."

She said the bus network would be redesigned to include high-frequency buses on every road so passengers could just turn up and go, and three extra green bridges would be constructed for pedestrians, cyclists and public transport.

Policy to cost $216m, Greens say

Ms MacMahon said the policy was costed.

"This has been written in conjunction with public transport experts and we have estimated that this will cost about $216 million to the State Government, which pales in comparison to the costs of some road-widening projects that are going on around Brisbane at the moment," she said.

"We've been proposing a number of revenue-raising measures overall in Queensland to make sure that big corporations are paying their fair share, so we can be investing in this kind of essential infrastructure that Queenslanders need.

"So making sure property developers are paying their fair share, making sure mining corporations are paying their fair share."

Public transport fares currently subsidised, Labor says

However, Labor Transport Minister Jackie Trad said while the Greens' policy would appeal to voters, it would place a huge impost on the budget.

"We currently subsidise public transport fares in south-east Queensland to the tune of $1.43 billion a year, so we are talking about significantly increasing the additional subsidy that the Government will provide to public transport," Ms Trad said.

"We've seen a stabilisation in terms of patronage and with some peaks and troughs, but what we do know is when you give people a reliable, convenient public transport system, they use it.

"That's why additional services like Cross River Rail are critical to the reliability and the future of our public transport network system."

Greens' policy may mean road funding cut, RACQ says

The head of public policy for peak motoring group RACQ, Dr Rebecca Michael, said the policy would likely mean less money for other parts of the Greens' transport budget.

"It has to come from somewhere," she said.

"While we would like to see a reduction in fares, we are also very conscious we would not like to see a decrease in funding for roads.

"We've seen the introduction of fairer fares in December 2016 and that was an 11.5 per cent drop in fares.

"Seventy per cent already is subsidised, so we'd be concerned that dropping to a $1 flat fare may be an additional burden on Queenslanders and may see a reduction in funding going towards our road network."