Ella, wearing "normal togs and a rash top", had spent around two hours bodyboarding in the afternoon.
Ella's twin brother, who had spent time in the water as well, did not come out in a rash.
Ms Morrison took her daughter to the medical clinic and was given a solution usually used for chicken pox, anti-histamines, camomile lotion and steroids as a backup.
Also known as the Sea Bathers Eruption, the culprits are the larvae form of tiny jellyfish with stinging cells, which are triggered by pressure or osmotic changes.
A saltwater shower works best, but freshwater is almost as good.
Bay of Plenty Polytechnic Marine and Environmental Management Group leader Dean Tully said Sea Bathers Eruption was more common in summer.
"They are really horrible, not bad initially but get worse and worse," Mr Tully said.
Life saving services and education manager Alan Mundy said the area has gone through a spate where there has been not many cases of bites and rashes associated with the ocean, but they "do come periodically".
He thought full moon periods "could have something to do with it", with an increase in reproducing marine life in the area.
Mr Mundy said fresh water showers after being in the sea could help.
A Mount Maunganui local advised, from personal experience, to take wetsuits or rash vests off before showering in cold water, or to take them off while still in the water.
The rashes usually arise when the sea water drains from wetsuits or rash vests, or when rinsing off in cold freshwater while still in wetsuits or rash vests.
Wearing bathing suits for a long period of time can make the Sea Bathers Eruption worse.
Rashes of the same nature have been found on Auckland beachgoers recently.
In a press release from the Auckland Regional Public Health Service, Medical Officer of Health Dr Simon Baker said warm weather and onshore winds can bring the jellyfish larvae to the beaches, and to rinse off quickly after swimming.
"A saltwater shower works best, but freshwater is almost as good.
"Then change out of your togs as soon as possible," Dr Baker said.
Proper washing of wetsuits, rash vests and togs is also important.
"Jellyfish can still sting even after they are dead, and it is possible to get stung weeks after initial contact if your togs haven't been washed properly," he said.
Ella is "fine now", but still has light pink marks where the rash was, which Ms Morrison expects to disappear after a week.
When asked if she would be returning to the Mount waves, Ms Morrison said although her daughter loves her swimming and loves the beach, she is now "a bit nervous taking her back to the beach".
Online:
* A post on the Mount Maunganui Noticeboard page on Facebook generated more than 100 comments, with many other residents suffering from the Sea Bathers Eruption.
