Unprecedented levels of contract instability in the A-League and the growing unevenness of the competition is proof the salary cap should be abolished, the Australian soccer players' union says.

Some 63.4 per cent of A-League players came out of contract at the end of the 2017-18 season, the highest level since Professional Footballers Australia (PFA) began recording that statistic five seasons ago.

The PFA's 2017-18 A-League report also found the gap in points between the first-place team and last-place team was consistently growing, the variety of teams playing finals was narrowing and the amount of games decided by three or more goals had increased.

PFA chief executive John Didulica said the union did not support the current salary cap model. The PFA is part of a working group looking at how to improve the A-League player payment system, with the current CBA expiring at the end of the 2018-19 season.