Australian prime minister Scott Morrison said players refusing the flu vaccination should not be allowed to take to the field as NRL training resumed on Wednesday.

The coronavirus-hit 2020 season moved a step closer to restarting on May 28 after all NRL teams returned to training midweek amid the COVID-19 crisis.

All eyes were particularly on Gold Coast Titans star Bryce Cartwright, who refused the mandatory injection ahead of the competition's resumption.

Asked about the situation amid criticism, Morrison told 2GB: "No jab, no play.

"When I was social services minister I started the 'no jab, no play' rule into the childcare facilities and I think the same rule applies [in the NRL]."

And we’re back!!! #NRL #ThruNThru pic.twitter.com/F4DJi2PUeQ

— Gold Coast Titans (@GCTitans) May 6, 2020

Cartwright has been the subject of criticism and his wife, Shanelle, leapt to the defence of the Titans second-rower on Tuesday.

"People have the freedom to say what they like, just like we have the freedom to choose which medical procedures we undergo, but ultimately the proof is in the pudding," she wrote via Instagram.

"Our kids are a picture of health. They've never had an ear infection, never had a chest infection or bronchitis, they have no neuro-developmental disorders or auto-immune disorders and are rarely ever sick.

"They've never had a round of antibiotics or any other pharmaceutical drug for that matter [yes, including Panadol]. As parents we do our best with what we know, for some, health comes in a needle and it works for them.

"What we're doing is going seemingly well seeing the state of their health. So if it's not clear, we're not anti-anything. We stand for medical freedom and the right to choose."

New Zealand Warriors, who have relocated from their Auckland base to the regional New South Wales town of Tamworth, resumed training on Tuesday.

However, all 16 NRL teams returned to training midweek after the season was postponed through just two rounds on March 23.