CANBERRA, Australia—One evening this week, fitness instructor Michael Montgomery tried to spur four dozen exercisers through a frenetic routine of lunges and jumping jacks by applying a tried-and-true prod—a Britney Spears tune.

"Gimme gimme more, gimme more, gimme gimme more," rang the lyrics of the 2007 song, which is called "Gimme More."

The words, however, weren't sung by Ms. Spears, but by an almost-but-not-quite soundalike cover artist identified as Mandy Brewer.

The song is one of scores of cover songs played by the outlet of the international Fitness First chain in response to new tariffs Australian gyms must pay if they play original, copyrighted recordings in classes.

In a ruling this week, the Copyright Tribunal of Australia ruled that music is essential to fitness classes, and artists should be paid accordingly. The tribunal raised tariffs for playing original artists' recordings to roughly 85 cents (A$1) per class participant—capped at about $13 per class—boosting the annual tab for the typical Australian fitness center from around $1,300 to more than $19,000.