A heavy rain warning has been issued for parts of the Bay of Plenty, with forecasters warning of possible flooding, slips and hazardous driving conditions from Wednesday afternoon.

MetService has placed an orange warning in force for areas west of Kawerau, from 3pm on Wednesday to 3am on Thursday.

During those 12 hours, 70 to 120mm of rain is expected, with intense downpours of 25 to 40mm/h possible in localised areas. Thunderstorms may also develop.

The severe weather is being driven by a slow-moving deep low forecast to move east across the North Island later this week, bringing widespread rain and strong winds to many regions.

In its latest weather update, the MetService said there is a low chance the warning could be upgraded to red, but conditions are being closely monitored.

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.

Rain is expected to develop across the Bay of Plenty from midweek, with the heaviest falls arriving on Wednesday. Showers, some heavy, are forecast to continue into Thursday before gradually easing.

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.