Saatchi promotes artists' work on its website and, when a buyer is found, provides a number of different services (e.g. payment processing and shipping). For this service, Saatchi charges a fee via its commission.
Saatchi Art is a US-based corporation registered in Australia and must charge GST on the e-service it provides to artists who reside in Australia. Saatchi is obligated to collect and remit GST to the proper tax authorities in Australia.
If you have an active Australian Business Number (ABN), please provide it in Studio to avoid GST withholding from your payouts.