Residents are being urged to prepare for heavy rain, with streams and rivers likely to rise rapidly and surface flooding and slips a risk.
Motorists are also being warned to take extra care on the roads as visibility reduces and surface water builds.
Further east, there is a moderate confidence that heavier rainfall could develop in eastern areas of the Bay of Plenty and northern Gisborne on Thursday morning as the system lingers.
Showers are forecast to continue across the Bay of Plenty into Thursday before easing.
The unsettled pattern is likely to persist into Friday as the low tracks slowly east, with the heaviest rain risk shifting further south and east by then.
Conditions are expected to improve from Saturday as the system moves away, bringing easing winds and rainfall and a reduced risk of severe weather heading into the weekend.
MetService said confidence in the exact impacts remains variable and people should stay up to date with forecasts.
Authorities are advising residents to clear drains and gutters, avoid low-lying areas during heavy rain, and drive cautiously.