Professional Footballers Australia (PFA) chief executive John Didulica hit out at A-League clubs amid reports several had stood down players without pay.
The A-League season was suspended last month due to the coronavirus pandemic, with a Newcastle Jets player and Wellington Phoenix staff member later testing positive for COVID-19.
Several clubs have reportedly stood down players without pay in a move that has angered the PFA.
Didulica said the game needed to work together to solve issues amid the coronavirus pandemic, accusing clubs of being opportunistic.
"Whether it is one club or all 11 clubs, our position on this will not change," he said in a statement on Wednesday.
"We will continue to fight for the players who have been dumped and preserve their legal rights, including challenging the stand-down notices and, where instructed, seek free agency on the basis of this breach of contract.
"We have significant concerns about the impact the decisions of club owners are having on the wellbeing of our members, many of whom now face long-term unemployment.
"It is entirely unnecessary for club owners to place their players in this situation when there is an alternative option – which is for the sport to work together – as we've seen across other mature and sophisticated sports.
"COVID-19 presents the greatest modern challenge to our organisation of sport. Actions not anchored in finding common solutions can only hurt. Short-term opportunism is coming at the expense of leadership and the long-term thinking that is now necessary."