The memo was sent out to all Te Whatu Ora nurses in Canterbury from Becky Hickmott, the executive director of nursing in Canterbury.
It said the region had "remarkably high" levels of sick leave compared to other big districts, and questions were being asked about why.
In the memo, Hickmott said under the national Te Whatu Ora system, managers could now compare sick leave levels across the country.
She said sick leave was particularly high on weekends and Mondays, and in places where staff were being called on to work in other areas to support staff.
Becky Hickmott said Te Whatu Ora was looking into the specific areas with sustained high sick leave, and seeking to understand the key driver.
Erica Donovan, the Canterbury regional chair for the New Zealand Nurses Organisation, said the memo implied nurses were taking sick leave when they did not need it.
"I think if this needs to be addressed to certain people that is absolutely fine, as an employer is well within their rights to do. But this has just come out as a global email when nurses are already stressed, and overworked, and being told not to do overtime but constantly getting texts asking them to do overtime."
She said the tone of the memo had left a bad taste in people's mouths.
"If anything a lot of our staff are probably burnt out and finding life really hard. So I wouldn't blame people if they did need to take time off."
Donovan said nurses feared this was connected with the current government push to save money within the health system.
Hickmott told RNZ that Te Whatu Ora was seeing higher levels of sickness among nursing staff and was concerned to see this happening consistently over many weeks.
She said the memo related to short notice sick leave (when nurses called in sick on the day of their shift), not sick leave in general.
In particular, the short notice sick leave for night shifts was an outlier with Canterbury sitting around 9.3 percent, she said. The next closest area of high sick leave usage was sitting at 7.8%.
"In light of this, we are undertaking some work to explore some of the drivers that might be behind the sick leave so we can look at how we can keep our teams well, how we can manage our sick leave levels, and also how we support those off for significant periods to return safely back to work.
"This is not about reducing costs, it is about staff and patient wellbeing."
