This PPCA is a non-profit organization that provides licenses to Australian businesses allowing them to play recorded music in a public setting.
The PPCA represents record labels and recording artists and distributes royalties to them when their sound recordings are used.
They do not represent the songwriters of a work, only the performers. Therefore, Songtrust would not collect these royalties, so you should absolutely think about getting involved with the PPCA if you own master recordings.
You can learn more at the PPCA's website here!