The Sanitarium plant in Christchurch is closed because of earthquake safety fears.
The food company says an engineering report has found there could be problems with the strength of a tower and parts of the factory building.
About 60 employees are at home on full pay for three weeks while the company waits for a second, more detailed report to establish what work needs to be done.
Sanitarium says its Auckland plant is working overtime in order to ensure there are sufficient stocks of Weet-Bix and Marmite in the South Island.
General manager Pierre van Heerden says he is confident that supplies will not run short though Sanitarium will probably have to go to a sister company in Australia to top-up some production.
He told Morning Report the company wants to keep the Christchurch factory open.