A submarine fibre-optic cable fault between Perth and Singapore could be why.
Australia's second largest DSL internet service provider iiNet is among a number of Australian providers affected by the issue that started on the SEA-ME-WE 3 cable on Sunday.
"Our link between Perth and Singapore is currently not passing traffic," an iiNet spokesman told Fairfax Media. "This traffic is taking an alternate path via Sydney to Hong Kong.
As a result, iiNet said customers may experience slower than expected download and upload speeds, latency and packet-loss to destinations in Asia.