The transport and logistics company said the mandate became effective on January 17.
In an update to the share market this morning, the company said preparations were underway for staff being away due to the Omicron outbreak.
The company also said revenue for 43 weeks of its financial year from April 1 to December 31, 2021, was up by 45 percent to $4.1 billion, from the prior corresponding period.
It has benefited from increased demand globally, with supply chain congestion showing no signs of slowing down.
"These financial results are not a surprise, rather a continuation of the increasing freight volumes and trends seen in our first-half results. They are also a reflection of the improving financial performance across our Asia, Europe and Americas businesses," the company said.
"International shipping and airfreight continue to be impacted by congestion, and with demand far exceeding supply, freight rates remain elevated."
Mainfreight said its operations in the Americas, Australia and Europe had been affected by staff absenteeism due to the Omicron outbreak.
"At the time of release, we have yet to see any impact in our New Zealand and Asia businesses, albeit China's border restrictions are tightening."
In New Zealand, the company said domestic transport levels remained elevated, with Christmas volumes the largest on record.
Overall, the company said it was a "satisfactory" year-to-date result.
"Navigating constraints due to Omicron-enforced absenteeism remains a priority for us. Meeting our current customer service requirements is a key objective through to year-end."
