Anna Francis McGirr appeared in the Christchurch District Court on Thursday, charged with taking marine life from the Pōhatu Marine Reserve in Banks Peninsula.
The British woman was fined $500 for fishing in a protected area.
She maintained there was no signage indicating the area was a marine reserve.
She and a group of other Brits were caught on a boat 10 minutes into their attempt to fish in the area.
McGirr's defence counsel said she was "a young woman, who had no idea what she was doing at all".
Her "essential naivety" ought to be taken into account was the argument.
Judge Mark Callaghan said "naivety is no excuse" and that McGirr's actions had threatened many protected species.
The boat's skipper was fined $750, but McGirr received a lesser fine as she spectated the act, but did not participate in it directly.
One of the signs near Pōhatu warns that fishing in the reserve can incur a fine of up to $250,000.
The Department of Conservation has been approached for comment on signage visibility.