Of the 38 cases notified in Northland between 2007 and 2015, 36 were associated with the farming sector, and 19 people were admitted to hospital.
Concerns have been raised that the recent spike could be related to farmers reducing vaccination programmes, bringing warnings, to dairy farmers in particular, to keep inoculations up to date.
Federated Farmers' Northland dairy chairman Ashley Cullen said farmers had a responsibility to vaccinate stock for human health and safety, as well as animal welfare.
However, he had not heard if this outbreak was associated mainly with dairying.
All stock farmers, possum trappers and others working with animals are advised to ask their doctor about leptospirosis if they feel unwell.
Humans get infected through breaks in the skin or through mucous membranes of the eyes, nose or mouth.
Water and wet conditions are key to the pathogens staying viable, whether contained in waterlogged land, urine splashes or even the puddles a farm dog drinks from, WorkSafe NZ agriculture programme manager Al McCone said.
"You don't have to come into direct contact with urine or infected tissue of an infected animal," Mr McCone said.
"Even a splash or fine spray of urine, or indirect contact with urine-contaminated water, such as water used to clean down a cowshed or stockyard, can spread the disease."
Prevention measures include controlling rodents, sweeping away surface water, personal hygiene and wearing protective clothing.
In a tough financial climate, dairy farmers might be tempted to let their vaccination programme lapse, Mr McCone said.
"But, with a bad case of leptospirosis, a farmer can be off work for six months. Farmers need to do those sums around the cost to them in that event compared to the cost of a vaccination programme."
The New Zealand Veterinary Association, DairyNZ, Federated Farmers and Rural Health Alliance Aotearoa New Zealand recently joined forces to call for farmers to stick to prevention programmes.
"Missing just one year of vaccinations puts you right back to square one," said Jenny Weston, president of the NZVA's dairy cattle veterinarians.
"If some cattle become infected during a lapsed vaccination period, they will continue to be a risk for infecting people even after vaccination has started again."
What is it?
• Leptospirosis is an infection caused by bacteria of the genus Leptospira.
• It is one of the most common diseases transmitted from animals to humans, with few known cases of human to human infection.
• All farm stock, rats, mice, possums, hedgehogs, less commonly domestic pets, soil, manure and unpasteurised milk can be sources.
• Hunters and meat workers can pick it up from urine-contaminated skins and infected renal organs.
• It is not a legal requirement for farmers to vaccinate stock but they must carry out all health and safety steps to prevent people on their land becoming ill or injured.
