The rāhui, or ban, is in place for an initial three years and is legally enforceable. Commercial fishing of any sort is also banned within the reserve.
Kaumātua Hugh Rihari, chairman of Te Komiti Kaitiaki Whakature i ngā Taonga o Tangaroa, said kūtai (mussels) at the Black Rocks had been declining for a long time but concerns had escalated in the past two years.
Now, three months after the ban was imposed, he was still getting complaints about people gathering mussels inside the reserve.
Rihari suspected some didn't know about the ban while others were deliberately ignoring it. There were also people who felt they had a right to take the shellfish because of their strong cultural connections to the area.
However, Rihari said the ban applied to everyone, even though it wasn't easy. Marae, for example, used to rely on mussels from the area to feed their guests.
After three years scientists would re-assess the health of the mussel population and, if necessary, the rāhui would be rolled over for another three years.
''We need to make sure we have mussels for our grandchildren. The only way we can do that is by not contravening the rāhui. We need to give the kūtai a rest and a chance to grow back.''
Marine conservation group Fish Forever has distributed signs and pamphlets to all boat ramps and marinas in the area.
Group member Dean Wright said the rāhui at Maunganui Bay/Deep Water Cove, which was also legally enforceable, had shown it could take a long time for word to get through to everyone.
■ Go to www.tepunamataitai.nz for a map with the reserve's boundaries and GPS coordinates, plus a study by marine scientist John Booth examining the mussels' decline. The causes are believed to include water quality, shellfish disease and harvesting pressure.