The Canterbury DHB is warning people to protect themselves against Legionnaires' disease ahead of Labour day.
The potentially fatal disease, a form of pneumonia, was caused by legionella bacteria that live in moist organic material.
People caught the disease by inhaling airborne droplets or particles containing the bacteria.
Canterbury medical officer of health, Dr Ramon Pink, said there were 23 cases of the disease confirmed in the region this year already.
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Last year, there were 39 hospitalisations from Legionnaires in Canterbury.
"Gardeners are at particularly high risk of catching Legionnaires' disease as the bacteria thrive in bags of potting mix and compost," said Pink.
He said there were usually spikes in cases in early November due to increased gardening activity over Labour weekend.
"The illness may be mild but can sometimes be fatal."
It is more common in older people, particularly if they smoke, have poor immunity or a chronic illness - however, sometimes even healthy young people have died from legionella pneumonia.
Pink said people should wear a face mask and gloves when gardening, open potting mix or compost bags with scissors carefully at the top of the bag, reduce dust by spraying water into the bag, work in a well-ventilated area, and thoroughly wash their hands afterwards.
Symptoms of the disease included dry coughing, high fever, chills, shortness of breath, muscle aches, headaches and diarrhoea.