Northern clubs will no longer be able to activate "hidden picks" during the draft in order to access academy players.

The AFL has ruled that the number of selections a club holds going into the national draft must match the number of vacant spots on their primary list.

The league hopes the change will create less incentive for clubs to trade down the draft in order to accumulate draft "points", as some did last year.

They wrote to clubs on Monday to advise them of the change to the bidding system, which will be effective at this year's draft.

The AFL also told clubs it would not alter the 20 per cent discount that applies to academy and father-son players for clubs matching bids.

Clubs are still permitted to do trade down the draft order, as they did last year, however they may now be more inclined to hold onto their early picks.

Last year, Brisbane went into the draft carrying considerably more draft picks than available list spots.

Their "hidden" picks were activated as they were required to use the points attached to more than one draft pick to match bids for academy players.

Greater Western Sydney and Sydney also traded down the draft order during the trade period in order to acquire extra points.

Sydney's willingness to move their first-round pick helped St Kilda and Essendon broker a deal for Jake Carlisle.

Clubs will still have a leeway of two picks, given they are allowed to finish the off-season with a primary list of 38-40 players.

Under the change, clubs that need to use multiple picks to match a bid for one of their players will be given a new pick or picks at the back of the draft.

That way, the extra selections will not impact on those of other clubs.

The AFL asked clubs for feedback on the bidding system earlier this year and received a range of options on whether or how it should be altered.

Many clubs hoped no change would be made for this year's draft, given they were able to trade into the future for the first time last year.

The 2015 draft order changed live on the night as the draft was unfolding, as bids were made and matched and as hidden picks were activated.