All three NRL trial matches in New South Wales are set to go ahead tonight despite the cancellation of a host of daytime sporting events due to the three-day heatwave.

The Sheffield Shield clash between New South Wales and Queensland will also continue at the SCG today with increased drinks breaks but Cricket NSW has cancelled all grades of Premier Cricket and recommended all associations in Sydney follow suit.

Explaining the decision, Cricket NSW doctor John Orchard said there was a significant difference in player support between the Sheffield Shield and other competitions.

"At the SCG we have medical staff in place that will be monitoring players who are showing any signs of symptoms and making sure no one with heat stress returns to play," Dr Orchard said.

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A day after the scheduled Jets-Victory A-League match in Newcastle was postponed until Monday, Football NSW cancelled all Saturday trial games for under-18s and below and shifted under-20s and first grade trials to evening start times.

With temperatures in the 40s tipped for some areas, there were widespread cancellations of school sports and the New South Wales Rugby League abandoned the start of its junior league competition.

NRL clubs deliberated yesterday before opting to proceed with their trial games.

Canterbury have pushed their Belmore NRL trial against Penrith back to 7.45pm (AEDT), while South Sydney and Manly clash at Campbelltown and St George Illawarra meet Wests Tigers in Wollongong also tonight.

"Player welfare is at the forefront of our thoughts and has been throughout our discussions with the NRL today," South Sydney general manager Shane Richardson said yesterday.

"We will be playing in four quarters, have unlimited interchange and have drinks breaks throughout the match, all which is designed to help the players."

The developments come after former NRL star Wendell Sailor claimed NRL trials in extreme heat were risking players' welfare.

Sailor told Triple M radio that trial games should be postponed until the heatwave subsided.

"It's ridiculous. The athletes and footballers these days push themselves as it is," he said.

"Some people say it's a necessary evil. I don't think so. It's a long enough season, the wear and tear and player welfare. It's not needed. It's ridiculous."

NRL chief executive Todd Greenberg said the league's heat policy put the decision on whether to play into medical experts' hands.

"The people who are most qualified to make decisions on player welfare are our doctors," Greenberg said.

Human athletes aren't the only ones suffering in the heat, with Racing NSW postponing today's race meeting at Royal Randwick until Monday but that date might also be deferred.