In legislation tabled on Thursday, Gaming Minister Joy Burch has left open the option of bringing forward the compulsory forfeiture of machines if clubs don't reduce the number of machines on the floor quickly enough.

She introduced legislation, to be debated in June, that will for the first time allow clubs to buy and sell poker machines between them. It also sets a ratio of 15 machines for every 1000 adults, to take effect after three years, giving clubs time to adjust. She estimates that will allow clubs 4785 machines in 2018, a reduction of 121.

Currently, clubs have 4906 poker machines; hotels and pubs have 68, which they will be encouraged to sell to clubs (and banned from getting new licences under the legislation); and 50 machines are held in a "pool" for reuse, for a total of 5024.

The poker machine trading scheme is expected to begin later this year, but given the strict limits on licence numbers, the government points out that clubs cannot buy machines from hotels or pubs or other clubs unless they are allowed to increase their numbers. The legislation allows clubs with fewer than 120 machines an extra 12 poker machines without a social impact assessment. Clubs with more than 120 will be able to increase numbers by 10 per cent, to a maximum of 20 extra poker machines without a social impact assessment.

But Ms Burch stresses that overall, the number of machines will begin falling from day 1, with clubs required to forfeit one machine for every four licences they buy from other clubs or hotels. If numbers don't come down fast enough she has the power to bring forward the ratio of 15 machines to every 1000 adults. When the ratio takes effect, at the end of three years if not sooner, machines will be taken compulsorily to reach the cap. Clubs with 20 or fewer machines will be exempt from the compulsory forfeit.

The cap will be based on the population at the time, and will be reassessed every two years, with a new cap set every two years. Government figures suggest population increases would allow 60 new machines a year. But the decision to release new licences is in the hands of the minister and Ms Burch stressed the ratio of 15 machines for 1000 adults is a maximum.

"This phased approach gives us a mechanism to reduce the number of gaming machines across Canberra and I am confident that it will get us to our target within three years," she said. "However, if we are not moving fast enough to reach our target, I will step in to move straight to phase 2 and immediately implement reductions."

Clubs will also be allowed to take machines off the floor for the first time. At the moment, if a club isn't using a machine, it is forfeit. But clubs will be allowed to quarantine machines for between one year and three years without losing the licence, to help them adjust to the new scheme.

The legislation tabled this week introduces a new tax regime for clubs to take effect on July 1. Overall, the amount of tax paid by clubs will remain about the same, but most clubs will get a tax cut, paid for by the big clubs. The government says 29 clubs will pay less, 13 will have no change and eight will pay more.