The number of people affected in a major outbreak of Legionnaires' disease in Auckland, which has claimed one life, has risen to 11.

A patient at Middlemore Hospital was diagnosed with the life-threatening disease today.

Legionnaires' is a form of pneumonia that can be life-threatening for people with weakened immune systems.

Since the outbreak six weeks ago, 12 people have needed hospital treatment and it's believed around 300 buildings in the city are affected.

Auckland Regional Public Health Service said a woman who had underlying health issues had died earlier this week.

Four patients who contracted the disease were admitted to North Shore Hospital. Two were discharged over the weekend, while the other two remain in serious but stable conditions.

Middlemore Hospital confirmed three other patients with Legionnaires' had been hospitalised there over the past six weeks, but they had all been discharged. One person was treated at Auckland City Hospital. It is unclear where the remaining two cases were treated.

Symptoms of Legionnaires' can include headache, diarrhoea, dry cough, drowsiness and delirium. It is treated with antibiotics, but most people who contract the disease are hospitalised.

ARPHS confirmed all patients diagnosed with the disease contracted it from a water source, which may include air conditioning systems in buildings.

There have been previous outbreaks of the disease from contaminated potting mix, but that is not thought to be a source of the Auckland outbreak.

Business owners and managers of buildings that contain an air conditioning tower or an industrial process that uses water to generate aerosols are being urged to immediately arrange shock-dosing of the systems.

"This is an urgent matter concerning the health of our population," said Dr Simon Baker, medical officer of health with the ARPHS.

Auckland Council has set up a rapid response team who are calling building owners to ensure they treat their cooling systems.

The council has also called water treatment companies which have already treated some buildings.

Baker said building managers had a responsibility to carry out the work, which means using a biocide to eliminate the Legionella bacteria.

More information is available via the council's central building information line on 09 353 9358.

Those who have concerns about their health should contact their GP.

- © Fairfax NZ News