Estimates from the Asthma Foundation suggest more than 13,000 children in Otago and Southland have asthma.
In the whole of the country, it says the number of children under 15 with asthma equates to about the population of Hamilton, more than 200,000.
Foundation chief executive Angela Francis said the estimates were based on earlier findings that one in four children had asthma and relating that to the 2006 census figures.
Ministry of Health figures show that the number of hospital admissions with asthma as the main cause in those aged 2 to 16 years totalled 4618, almost half of the total number for this type of illness.
The foundation said it was the leading cause for hospitalisation among children aged under 5.
In Maori, it was the leading cause of hospitalisation across all age groups.
Ms Francis said it was not just about numbers, but the effect on children's everyday lives.
The foundation was committed to alleviating the fear, pain and disadvantage children with asthma faced.
This included "sudden, scary trips to hospital", wheezing, breathlessness and days off school.
To promote better management of the condition, the foundation encourages those who have children with asthma to ensure they have an asthma plan which they can fill out with their child and a health professional.
This is designed to ensure regular use of preventive medication rather than people waiting until there is a problem and then using relieving medication.
The foundation does advise, however, that anyone who is scared about their child's breathing should dial 111.
In 2006, the last year for which statistics are available, 132 people died from asthma in New Zealand.
The foundation estimates 80% of asthma deaths would be preventable if the disease was managed properly, although there are still instances where people get severe unexpected attacks after decades of asthma-free living.
• Asthma Awareness Week is April 27 to May 3. Balloon Day is on Friday, April 29, with the message that one in every four 4 children struggles to blow up a balloon, because "asthma won't let them".
Asthma
Symptoms are:
> Coughing, particularly at night and after exercise
> Breathlessness
> Wheezing or noisy breathing
> A tight feeling in the chest
Symptoms may occur suddenly as an " attack" or may be with you all the time. You may experience one of these or a combination. Often those with the condition also have hay fever or eczema and a family history of these conditions.
> If you are concerned your child may have the condition seek medical help so any triggers of the illness can be determined and ways of controlling it established.
Numbers of children estimated to have asthma ( based on 2006 census):
Otago - 8527
Southland - 4816
New Zealand - 215,923
Source: the Asthma and Respiratory Foundation of New Zealand
