In the last four years, the big teams from Sydney, Melbourne and Brisbane have dominated the A-League finals with only cameo appearances from their rivals.
And Sydney FC twice ran away with the Premiership with 44 more points than the bottom-placed club.
In the their new annual report, Professional Footballers Australia says the root cause is the big teams' increasing ability to spend more on wages – and smaller clubs unable to keep up any more.
"Any way you cut it, the A-League has become less competitively balanced during the period with the current ten clubs," says the PFA Annual Report.
"Across seasons, within seasons and within matches, the same teams are winning more consistently and by bigger margins.
"The points spread between the top and bottom of the ladder has grown. Within individual matches, the share of matches decided by three or more goals has (also) generally grown.
"And fewer clubs are sharing in the success. The variety of teams finishing in the top positions has narrowed.
The five teams from the biggest markets have reached the finals 17 out of a possible 20 times in the past four seasons, while the other five clubs only made 7 out of a possible 20 finals berths."
The PFA warns that the big-spending clubs can afford to stretch their salary cap and marquee signings to create more entertaining and successful clubs.
But the clubs from areas of a smaller population and tighter budgets can't compete because the FFA grant doesn't allow them to be competitive.
As a result, says the PFA, the salary cap no longer levels the playing field as originally intended and just allows rich clubs to prosper.
"Because the size of the annual club grant has not kept pace with the ambitions of the well-capitalised clubs, the smaller clubs have been left well off the pace," says the PFA.
"Newcastle Jets’ Cinderella story this season was all the more compelling because it bucked this trend.
"But even so, their rise up the ladder was fuelled in part by significantly increased investment in their playing squad from the previous season.
"Growing the club grant depends on the league’s administrators maximising the commercial potential of the league and investing a fair share of that revenue back into clubs.
"Arguably, neither of these has happened.
The PFA adds: "What we have now is a league that is no more or less competitively balanced than the average European league, but one in which players and clubs suffer the unwanted side effects of a labour market model which has a primary purpose of promoting competitive balance.
"It is the worst of both worlds."