Crabtree said peak intensities may reach 20 to 30mm per hour, possibly more, during Wednesday morning and thunderstorms and strong winds were also possible.
"The heaviest falls will be tomorrow morning but it will start today and it looks like there will be waves of it. Sometimes it will be quite heavy, then less heavy," she said.
Crabtree said people should be cautious as the rain would likely cause streams and rivers to rise rapidly and flooding and slips were also possible. However, most of the rain was expected overnight "so hopefully there won't be too many people travelling on the roads".
"But if someone is driving in this heavy rain they do need to take care. There could be surface flooding on the roads. Also, there may be some thunderstorms and strong winds. It is the whole shebang.
"During tomorrow it should ease up during the morning. We will also see a strong northeasterly flow, especially for people on the coast. It will be particularly windy but it will ease off. It will be break tomorrow before another of showers."
Despite the wet and windy forecast, the overnight temperature was expected to reach a low of 17C in the Bay "so it might be quite humid".