Westland Milk Products says its tankers are still managing to collect milk despite slips and trees blocking many of the region's roads.
It says communications have been cut off to most of its Buller area farms and some in South Westland, but tankers are getting through where there is access.
Operations general manager Bernard May said the Hokitika factory is operating as normal and milk is being processed.
The company is trying to contact affected farms via radio alerts, the internet and in person where it can.
May said he expects most farms will have their silos cleared today and if silos are full, milking should be delayed as long as possible until the milk can be collected.
If disposal of milk is unavoidable, he said farmers should follow prescribed procedure for milk disposal to effluent holding ponds.