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The 820 Visa (onshore) is for partners already in Australia, allowing them to stay while applying for permanent residency. The 309 Visa (offshore) is for those outside Australia and leads to permanent residency through the 100 visa. Choose the 820 if you're onshore, and the 309 if you're overseas. Both visas eventually lead to permanent residency. To know more read the blog by Ozee Migration on 820 Visa vs. 309 Visa: Which Partner Visa Option Suits You Better?