Fisheries officers have called illegal set netting in the Auckland area a "scourge".
They say staff are retrieving and dismantling around six illegal nets a week, and that the nets are frequently set up in closed areas.
Staff are also urging recreational anglers using rods to make sure they know the rules, saying there's "no excuse" for taking undersized snapper.
Newshub went along with Fisheries officers on a patrol of the Hauraki Gulf.
In total, 23 inspections of recreational vessels were carried out during the half-day operation.
They found three non-compliant vessels, plus fishers with undersized snapper.
Nine years ago, the legal minimum size for snapper in the fishery zone known as Snapper 1 increased from 27cm to 30cm. Snapper 1 incorporates the Hauraki Gulf.
However, some who spoke to fisheries staff didn't appear to know anything had changed.
North Harbour District team leader Glen Blackwell told Newshub that's "not an excuse".
"They should [know the rules] and they need to educate themselves and there are tools available to do that."
Blackwell said that should start with downloading the Fishing Rules NZ app, which lists all species and size limits, plus other rules.
For the three anglers who breached the rules, the details of each person were noted and each fisher was given a formal written warning.
That means the details of the boat and owner are kept in Fisheries New Zealand's database and should further offending be picked up, it will mean an instant $250 fine. In more serious cases, vessels can be seized.
The warnings were issued as it did not appear any of the breaches identified were intentional, flagrant rule-breaking, Blackwell said.
However, that sort of offending is occasionally picked up, according to Blackwell's colleague and senior officer, Monique Scott.
"People just going out with no regard for the rules and just filling up chilly bins, getting more than what they should, and smaller size as well which is just heartbreaking for us," Scott told Newshub.
Fisheries officers also encountered divers who had mountains of kina on their vessel. The men said they had a customary permit to take 500 kina but did not have the permit with them on their vessel.
"It should be onboard to be produced, so that's why we've followed them back to sight the permit to make sure it's current and correct," said Matt Parry, another senior officer.
After tailing the vessel back to the carpark at Half Moon Bay, the divers produced the correct paperwork.
Figures show the majority of recreational fishers do adhere to the law.
Fisheries officers do around 25,000 to 30,000 inspections of recreational boats every year and the compliance rate is high. Around 94 percent of people officers interact with follow the rules.
But there's another emerging problem when it comes to recreational fishing in the country's most popular fishery.
That is illegal set nets.
"It's for us, a bit of a scourge, particularly around the Auckland area," Blackwell told Newshub.
Some are oblivious about what the rules state, he added.
Blackwell said nets are being set in closed areas like the Tāmaki estuary, they're not properly marked, or incorrect mesh sizes are used.
And sometimes people catch more than the daily limit of 20 fish per person.
"We spend a lot of time retrieving illegal nets and disposing of them. In the Auckland district we're probably picking up around half a dozen nets a week," Blackwell said.
"If you don't know the rules and you don't get it right and you come across us, then you're going to have a problem."
It's a problem that could seriously curtail a summer session on the water.
