MetService has issued a red heavy rain warning for parts of Marlborough and Canterbury, with forecasters warning of dangerous river conditions, significant flooding and slips.

The warning covers the area between Ward and Hanmer and southeast of the Awatere Valley, including the Kaikōura ranges, and is in force for 35 hours from 4pm Monday until 3am Wednesday.

MetService said 250 to 350 millimetres of rain was expected, with totals possibly reaching 400 millimetres.

Peak rates of 15 to 25 millimetres an hour were forecast for Monday night and during Tuesday morning and afternoon, especially about the ranges.

The freezing level was expected to sit at about 1600 metres, with snow likely above 1200 metres.

Forecasters said conditions would disrupt travel, make some roads impassable and isolate communities.

The National Emergency Management Agency advised people not to enter floodwaters and to avoid travel.

“Act quickly to self-evacuate if you see rising water. Be ready for power and communications outages,” the agency said.

The warning was issued at 11.43am on Monday.