Dr Ortega said heavy rainfall could impact the spread of cryptosporidiosis or giardia cysts - which lived in rivers and lakes and caused illness when inhaled - and faecal bacteria.
"These micro organisms need and thrive in humid ambient," he said.
Dr Ortega said high temperature and inadequate storage of food as well as poor hygiene also contributed to the increased risk of infection in households and the workplace.
Principal of Hora Hora Primary School said tummy bugs had taken about six teachers and about three children out of action in each of the 17 classes since school started back two and a half weeks ago.
"We've noticed quite a few kids away with sore stomachs too, not an epidemic proportion but quite a few. We've had to put relievers in but we've been lucky because they've been available at this time of year.
"The advice is always to keep the child home no matter what the illness is," he said.
Mr Newman said most teachers and students were only absent for one or two days.
Dr Ortega said the impact on hospitals had also been minor.
"These gastrointestinal diseases are often self-contained and resolve after a few days. Sometimes they will require antibacterial treatment and only in a few number of cases will more medical intervention be necessary," he said.
Dr Ortega said there had been 18 hospitalisations for stomach bugs during the last 14 months.
His advice for people suffering stomach bugs: * Stay hydrated by drinking plenty of water, even if you can only sip it, to replace any fluids lost through vomiting and diarrhoea.
* Rest as much as possible
* Eat when you feel up to it – sticking to small, light and non-fatty meals at first (bland foods such as toast, crackers, rice and bananas are good choices)
* Avoid alcohol, caffeine, fizzy drinks and spicy and fatty foods because they may make you feel worse
* Contact your GP if your symptoms are severe or don't start to improve in a few days
* Stay off work or school until at least 48 hours after the last episode of diarrhoea
* Make sure everyone in your household washes their hands with soap and warm water regularly – particularly after going to the toilet and before and after preparing food.
* Do not prepare food for other people
* Clean surfaces, toilet seats, flush handles, basins and taps frequently
* Make sure everyone has their own towels and flannels
* Wash the laundry of the infected person on the hottest washing machine setting