The AFL have introduced a raft of changes to cope with extreme weather in this weekend's round of pre-season matches, but they will not budge on their controversial interchange restrictions.

The cap of 16 interchanges per quarter - a trial rule for the NAB Cup - has prompted a constant wave of complaints from coaches, including warnings the restriction could adversely affect players' health and potentially lead to legal action.

The AFL's decision came after West Coach John Worsfold and Fremantle counterpart Mark Harvey displayed a rare show of unity between the West Australian AFL rivals by making a joint call for the interchange restriction to be relaxed for their clash in Perth on Sunday.

"For our doctor and our conditioning staff they would say it can be dangerous if a player is not coping with the heat and he has to stay out on the field," Worsfold said.

"I ... was quite happy to trial the 16 interchanges per quarter.

"But the AFL has had in place for a few years an extreme heat policy, and one of the things in there tells coaches they need to rotate players off the ground more to assist them."

Harvey was even more vehement in his opposition to the cap.

"Clearly the coaches had a meeting with the AFL and every one of those coaches in that meeting was not in favour of limiting the rotations on the bench - every coach," Harvey said.

"The modification we are looking for is either an extension of the bench or unlimited rotations."

But a relaxation of the interchange cap was the one thing notably absent as the league announced modifications to protect players, including delayed start times, shorter quarters, longer breaks and extra water carriers.

Forecasts are for monsoonal rain and high temperatures in Darwin, hot weather to affect games in Perth and Adelaide, while warm weather is expected in Melbourne, Geelong and the Gold Coast.

The league responded by applying varying match conditions to each game and said they would continue to monitor conditions.

The most drastic changes were made for games in Darwin and Perth, where quarters were shortened to 15 minutes plus time-on, down from 17-1/2 minutes, breaks between quarters were extended and extra water carriers will be permitted.

The start time for Sunday's match between West Coast and Fremantle at Subiaco was set back by 30 minutes to 7.10pm local time (2110 AEST).

Perth is expected to have a maximum temperature of 33 degrees on Sunday.

For Friday night's Darwin clash between the Western Bulldogs and North Melbourne, monsoonal showers are expected, while the forecast maximum is 31 degrees.

Interestingly, the rules were not altered to the same extent for Saturday evening's match between Port Adelaide and Carlton at AAMI Stadium, despite a forecast maximum temperature of 36 degrees.

Permission for extra water carriers to be used was the only modification for the Adelaide match.

Saturday's Geelong-Melbourne match at the Cats' Skilled Stadium, where there is a forecast maximum of 26 degrees, has been set back one hour and extra water carriers will also be permitted there.

No changes were made for Friday night's St Kilda-Richmond meeting at Telstra Dome, the Essendon-Brisbane clash at Carrara on Saturday night or the Hawthorn-Sydney encounter in Launceston on Sunday.

"We have taken a range of measures across four games this weekend due to the expected weather conditions in a number of locations, in accordance with the AFL's heat guidelines," Anderson said.

"We will continue to monitor these locations and each venue closely throughout the pre-season period."

The pre-season competition already includes measures to help players cope with heat in every game.

Quarters are two-and-a-half minutes shorter than the 20 minutes during the premiership season, while eight players are allowed on the bench, rather than four.

Brought to you by AAP © 2024 AAP