The cost of illness to New Zealand employers is likely to be $2 billion a year across the whole workforce, or more than $1500 per employee, a new study has found.

Surprisingly, the greatest part of these costs is not incurred by the sick people who stay away from work, but by the productivity lost from the sick people who still go to work, according to the research conducted for health insurer Southern Cross.

The study found the average time off work due to illness was 4.2 days a year, while the average number of days where employees went to work when they were too sick to be fully functioning and productive was 11.1 days.

"If you assume an unwell employee is half as productive as they normally would be, the cost to employers for a staff member on the average wage is around $900," said Southern Cross chief executive of health insurance, Peter Tynan.

"That's a higher average annual cost to employers than days off work, which is around $700 per employee."

If the numbers were multiplied out across the entire workforce, it would result in a $2 billion impact on productivity, he said.

Mr Tynan said the research pointed to the benefits of initiatives to enhance workforce health, which more employers were taking.

The study found two thirds of employees were offered some health and wellness interventions in the workplace - the most common being flu vaccinations, workstation assessments and stress support.