AgResearch is warning farmers to look after animal welfare and make sure cows have enough shade during hot weather.
New Zealand has had several days with record-breaking temperatures this summer and scientists say this raises concerns for the welfare of stock.
Research shows that when the air temperature reaches 21°C and humidity is more than 75 percent, it can affect the cow's behaviour and milk production.
Senior scientist Karin Schutz said this year had been particularly hard for cows.
"Especially if the cows don't have any opportunity to cool down, if they don't have shade or the farmers are not using sprinklers on farm then the animals can really struggle in this hot weather that we've had.
"A large animal like a lactating cow generates a lot of metabolic heat, and while it will increase its respiratory rate and sweat like a human being, it can struggle in especially warm conditions to lose the heat."
When asked if the increase in irrigation was a problem because it could lead to farmers removing trees, Ms Schutz said she could understand why farmers chose to chop trees down, but for the welfare of the cow's trees were important.
"Obviously I would like to see more trees on farms but there are other things farmers can do ... Any type of cooling, sprinklers or shade, lots of drinking water, and try to reduce physical activity.
"We do see a decline in milk production in summer unless the cows are kept cool."
She said farmers could reduce heat stress by grazing cows closer to the dairy shed, milking later in the afternoon when the temperature had dropped, and feeding supplements at night so the extra heat generated by digestion occurred at the coolest time.
"They often crowd together and they can stand and have high breathing rates and high respiratory rates.
"When you see the animal starting to drool and open-mouth panting, it's a sign it is in distress from the heat."