Following extreme drought during the 2019–2020 bushfire summer, the eastern part of Australia suffered from a week-long intense rainfall and extensive flooding in March 2021. Understanding how much water storage changes in response to these climate extremes is critical for developing timely water management strategies. To quantify prompt water storage changes associated with the 2021 March flooding, we processed the low-latency (1–3 days), high-precision intersatellite laser ranging measurements from GRACE Follow-On spacecraft and determined instantaneous gravity changes along spacecraft orbital passes. Such new data processing detected an abrupt surge of water storage approaching of ∼5 mm measured by a network of ground GPS stations. This was the highest speed of ground water 60–70 trillion liters (km3 of water) over a week in the region, which concurrently caused land subsidence recharge ever recorded in the region over the last two decades. Compared to the condition in February While these two events together replenished the region up to ∼80% of the maximum storage over the last 2020, the amount of recharged water was similar but the recharge speed was much faster in March 2021. two decades, the wet antecedent condition of soils in 2021 was distinctly different from the dry conditions in 2020 and led to generating extensive runoff and flooding in 2021. Plain Language Summary The monthly mean snapshots of global gravity field and surface mass variation (“mascon”) from the GRACE and GRACE Follow-On spacecraft missions are problematic to accurately quantify abrupt water storage changes and flooding by intense rainfall. This study demonstrates a new use of the GRACE Follow-On data to measure immediate water storage changes by computing instantaneous gravity change along spacecraft orbital passes. This new application is also shown for low-latency (a few days) data processing to assess surface mass changes immediately after extreme events. The results found that the eastern parts of Australia experienced the highest speed of ground water recharge ever recorded in the region and the wet antecedent condition of soils yielded extensive flooding in 2021.