Health New Zealand said there were 10 cases of legionnaires' disease in the year to March, up on three for the same period last year. Eight of those new cases were strongly linked to spas.
National Public Health Service medical specialist Matt Reid said ESR tested those with the disease.
"The same type of legionella bacteria was detected in both the infected person and in water samples taken from their spa pools. This means that infected water was the most likely source of infection."
In one of the cases, a more detailed investigation found the patient had bathed in a spa that had not been properly cleaned or maintained over time, he said.
The increase prompted a warning from Health NZ for people to regularly clean, disinfect and maintain their pools.
People could catch the disease by inhaling airborne droplets containing legionella bacteria. Symptoms could be mild, but could also be a severe flu-like illness or pneumonia that was sometimes fatal.
Legionnaires' disease was more common in people over 50, smokers, or those with a weakened immune system or other lung conditions.
The bacteria liked to live in moist conditions and could also be found in soil, compost or water systems, Reid said.
Gardeners should use a face mask and gloves when opening bags of soil and do any planting outside in a well ventilated area.
