The answer changes throughout the year! Western Australia does not observe daylight saving time, but New South Wales (Sydney) does. This creates varying time differences:
During Sydney daylight saving time (October to April): Perth is 3 hours behind Sydney.
During Sydney standard time (April to October): Perth is 2 hours behind Sydney.
Business hours overlap well! 9:00 AM - 5:00 PM Perth is 11:00 AM - 7:00 PM Sydney during DST, or 11:00 AM - 7:00 PM in winter. Good overlap for cross-country calls.
Despite being in the same country, Perth and Sydney are nearly 4,000 km apart - further than London to Moscow! The transcontinental flight takes about 4.5 hours. Western Australia's mining industry drives significant business communication with Sydney's financial sector.