The NRL will allow players who have a conscientious objection to flu vaccination to sign a revised waiver permitting them to train and play in the 2020 season.
The league's announcement came after several players objected to having a flu vaccination as a requirement ahead of the NRL's planned restart on May 28 following the coronavirus shutdown.
Gold Coast's Bryce Cartwright said he would not have a flu shot, while Canberra Raiders forwards Sia Soliola, Joe Tapine and Josh Papalii were told to stay away from training on Wednesday after they refused to sign an unmodified version of the vaccination waiver.
The NRL said its players were required to be vaccinated for flu, but could be exempted in exceptional circumstances such as on religious, medical or conscientious grounds.
"These (biosecurity) protocols have been reaffirmed to clubs and players today, including the requirement for flu vaccinations for all players and staff," an NRL statement said.
"The protocols allow for exemptions to vaccinations under compelling circumstances, including requiring players to sign a release. Until an NRL-approved release is acknowledged and signed by players, they will not be permitted to train."
The NRL said 97 per cent of players and staff had already been vaccinated.
Prime Minister Scott Morrison applied pressure to NRL players on Wednesday, declaring they should abide by a "no jab, no play" policy.
Home Affairs Minister Peter Dutton backed up Mr Morrison, saying the Federal Government's decision to allow the Warriors into Australia was based on the NRL's promise to adhere to a set of conditions, including seasonal flu shots.
"The conditions were obvious and the commitments were made by the NRL before a decision was made to allow them to go ahead," Mr Dutton told 2GB radio.
"We provided support, obviously, for the players to come from New Zealand, and we did that based on the health advice. And the health advice was based on the commitments given by the NRL. So I think it's pretty clear-cut."
Cartwright said in a social media post that he would not be forced into getting a flu jab.
"I won't be bullied into making decisions that could impact my health and the health of my family," he posted on social media.
"Giving us the ultimatum of get the shot or be stood down is coercion and leaking private medical information is illegal.
"As for me being the first and apparently only one declining the shot is bullshit and far from the truth."
Mother's Day visits off for NRL players
The NRL has informed players that Mother's Day visits to family would not be permitted, as they were not exempt from its COVID-19 measures.
Its biosecurity advice outlined that such visits were a risk as the NRL edges closer to its return date later this month.
Sydney Roosters veteran Brett Morris said he had no problem with the decision, as players could still communicate with their mothers from afar.
"At the end of the day, we've all got phones. They've all got cameras on them. You can FaceTime," he said.
"She doesn't need to see us face-to-face to know how much we love her."