Farmers and trampers near Gisborne have been warned that torrential rain forecast for today and tomorrow could become a "significant event", causing flooding and slips.

Met Service has issued a heavy rain warning for the hills and ranges of Gisborne and Hawkes Bay, north of Wairoa, for the 24 to 27 hours from 9am today.

Up to 180mm is expected to fall in that time, with another 40 to 70mm expected through to Monday morning.

The heaviest falls are likely in the coastal ranges north of Gisborne tonight or tomorrow morning when thunderstorms are expected to dump up to 30mm of rain an hour.

"People in these areas are advised that this is likely to be a significant event," said Met Service forecaster Ian Miller.

"Streams and rivers are expected to rise quickly, and there could be surface flooding and slips on roads in hilly country."