Two Auckland men who bashed a seal pup caught up in their fishing net so badly it had to be put down have escaped serious penalties.

In Waitakere District Court on Tuesday, Matthew Botica, 23, was discharged without conviction and Tomoare Angoika, 40, was sentenced to 100 hours' community work after earlier admitting a charge of ill treating an animal, Fairfax reported.

The seal pup got caught up in their net while the men were fishing in a west Auckland estuary last year.

Botica hit the seal with an oar four times and Angoika once, knocking it unconscious.

Passers-by called police and the seriously injured seal was taken to Auckland Zoo where it was later put down.

Defence lawyer Steve Bonnar said the case was not one of malevolence and if they had left the seal in their net it would have drowned.

"They made the mistake of thinking their only option was to strike it to stop it struggling."

Judge Claire Ryan described as stupid the idea that they could stun the seal and render it motionless.

Botica was ordered to pay the zoo's vet bills of $665.

Under the Animal Welfare Act, ill treating an animal carries a maximum of 12 months imprisonment or a fine up to $50,000.