Miss Flynn said many people thought they were doing a good thing by feeding the ducks and not wasting the bread "but mouldy bread is the worst for them".
She said feeding them bread caused deficiencies which resulted in poor immune systems leading to bowed feet, calluses and infections.
Another condition they can develop is called angel wings, which is an incurable wing deformity preventing their ability to fly making them vulnerable to predators.
Miss Flynn said she started caring for the birds when she was just 8 years old.
Now aged 26 she does not plan on stopping any time soon.
Currently, she has about six birds at her home but has taken in about 25 since the start of this year.
She said most of the swans were rescued from Anderson Park while ducks throughout wider Hawke's Bay were also brought to her.
Napier SPCA senior animal attendant Joy Walker said the public brought birds wrapped in towels and in boxes to the SPCA nearly every week. She said locals also called regularly, alerting them to sick birds.
"Liv is great, she takes them in, feeds them up and releases them back."
She said Miss Flynn had more space than the SPCA had to deal with the large numbers brought in.
Miss Flynn said she was already starting to see the impact of feeding birds bread, but in a few years the damage would become clearer as the ill health was passed down the generations.
She said this was not a new issue - bread had always been bad for birds.
"People can still feed the birds at the park but they need to take along birdseed rather than bread," she said.
"You can get a big bag for just $2," she said.