MetService says heavy rain pounding the top of the South Island could reach 1 in 50-year levels.
More than 330mm has been recorded in the ranges inland from Golden Bay since late on Tuesday afternoon.
Forecaster Andy Downs says the rain is not expected to ease until Thursday lunchtime, by which time the rainfall will have reached severe levels.
Mr Downs says slips and further flooding are likely throughout the Tasman District.
Warnings are also in place for Mt Taranaki, Northland, the north of Auckland, Coromandel Peninsula, Bay of Plenty and Tongariro National Park from Wednesday afternoon or evening.