> EST, according to Google, is the east coast of America when daylight savings is not happening. For Sydney and Melbourne, you want AEST / AEDT / AET

Although AEST/AEDT are probably more common-in part to avoid confusion with American EST/EDT-people use EST/EDT for the Australian time zones too - random example: https://www.support.transport.qld.gov.au/qt/systemmaintenanc...

Some computer systems (probably designed by Americans) want timezones to have abbreviations but insist they can only have three letters, so in those systems the Australian timezones have three letters. I definitely remember seeing EST meaning UTC+10 on Unix systems before