The massive storm, currently a category three tropical cyclone on track to hit Norfolk Island on Saturday, is likely to then impact New Zealand's North Island.
Meteorological models suggest Gabrielle will reach New Zealand as an extra-tropical low but one that will still pack a mighty punch.
Gabrielle could carry 140kmp/h winds across much of the North Island - including Auckland, Northland, the Coromandel and Tairawhiti - and up to 300mm of rain in a few days.
MetService head of weather communications Lisa Murray said Gabrielle could be New Zealand's biggest weather event in at least two decades.
"It's the most extreme storm I have seen, especially given its tracking over our main city," she told AAP.
The concern is heightened as cyclone tracking has Gabrielle passing through Auckland, the scene of record single-day rainfall a fortnight ago which killed four people and caused roughly $NZ1 billion ($A0.91 billion) worth of damage.
"Auckland is still in recovery from phenomenal flooding by slow-moving thunderstorms," Ms Murray said.
"Auckland, Northland, the Coromandel Peninsula, the Bay of Plenty - these places are already very sodden areas. Their water table is already very high.
"The impact of additional rainfall is expected to cause more flooding, landslips and there's the added complication that the tidy-up is still going on ... debris could block drains or in strong gusts, 140kmp/h, could become flying missiles.
"It's very important people prepare for this event."
The storm is likely to reach Northland on Sunday before passing across the North Island, with its biggest impact on Auckland - home to one-third of all New Zealanders - to come on Monday.
Another 50 defence personnel joined the clean-up crew in Auckland on Friday, helping transport flood-damaged and contaminated household goods to tips.
Emergency management authorities are prepared for the storm - as opposed to the record rain last month - with a state of emergency extended to allow the government to respond appropriately.
"The changing nature of the cyclone means everyone in Northern, Central and Eastern North Island should be on alert ... the government is taking this very seriously and is ready to respond to keep people safe," Emergency Management Minister Kieran McAnulty said.
Air New Zealand has offered flexibility to New Zealanders who don't want to travel during the storm and is likely to face wind-related cancellations.
Ms Murray said Gabrielle's track would determine how destructive it would be.
"We are confident it is coming to the north of New Zealand. At that point it does take a slight curve in the track," she said.
"At that point they usually intensify in strength ... that is the key component for how much it will impact New Zealand."
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