The Broncos on Wednesday announced four players had tested positive for the virus but the quartet were all asymptomatic.

They have since returned to training given their tests were taken over the Christmas period and they had complied with the Queensland government's mandatory seven-day isolation.

The rest of the squad was sent for testing on Tuesday, with 16 additional players recording positive findings.

They have all been ordered to isolate at home for the next seven days and will be allowed to return to training once they have been symptom-free for 48 hours.

Despite the large numbers of positive cases, the remaining 22 members of Brisbane's squad attended training on Thursday.

Every member of the Broncos playing group is double vaccinated.

"Our main concern is the health, safety and wellbeing of our players and staff, and we are fully focused on ensuring they are looked after while isolating," club chief executive Dave Donaghy said in a statement.

"The COVID testing measures are in place to protect our people and our program and these results show that the system is working.

"These cases have been picked up early, allowing us to deal with them promptly and effectively, and then move forward with our pre-season.

"We've taken a proactive approach to managing this challenge, which will have many turns over the coming season. Our goal as a club is to tackle each challenge as they arise.

"As the spread of COVID grows across the community, we will continue to work closely with our players, staff, the NRL and the Queensland government."

The Broncos latest comes a day after premiers Penrith confirmed "a significant number of the club's NRL players and staff are currently self-isolating and will not be available for the recommencement of pre-season training this week".

A Panthers statement said those affected have either tested positive for coronavirus or been identified as a close contact of a positive case.

Australian Associated Press