Cronulla halfback Nicho Hynes and prop Toby Rudolf have been ruled out of Thursday night's NRL top-four clash with Melbourne after testing positive to COVID-19.

Hynes returned a positive rapid antigen test on Wednesday morning after arriving at training.

The club later announced both he and Rudolf had tested positive on a PCR but all other players had returned negative findings.

Hynes won't be able to assume his role in the extended NSW State of Origin squad this week with Chad Townsend brought in to replace him.

Braydon Trindall will step into the halves now that Hynes has been ruled out and Braden Hamlin-Uele appears to be the most likely replacement for Rudolf in the front row.

Hamlin-Uele has been sidelined since tearing a pectoral muscle in their previous clash with Melbourne earlier this year.

The absence of Hynes and Rudolf will be a big blow for Cronulla particularly given Melbourne are low on troops and coming off an ugly loss to Manly.

Craig Bellamy's side will be without Queensland State of Origin duo Harry Grant and Cameron Munster plus injured pair Xavier Coates and Reimis Smith, while Felise Kaufusi remains away on personal leave.

For Craig Fitzgibbon's Sharks, the next few weeks shapes as the biggest test of their finals credentials.

The Sharks sit fourth on the ladder and, as well as hosting Melbourne on Thursday, play other top-four sides North Queensland and Penrith in the following two weeks.

"It is a big challenge and I know they're missing a couple of players but they're still not all that understrength," Fitzgibbon said.

"They're a club that's built on a system where if anyone leaves someone else comes in and fulfils that role.

"Historically speaking they haven't lost too many back to back.

"We've had to fight for those victories and teams have played well against us.

"We've got the right to be in the position we're in but I have no illusions as to what's coming up."

Meanwhile, Briton Nikora has signed a contract extension that will keep him at the Sharks until at least the end of 2025.

The second-rower made his NRL debut for the Sharks in 2019 and has since played 76 first-grade games.