Marine conservationist and dive pioneer Wade Doak said that despite the "slog", more people now go to the pools during the high tides, which is when the swell was likely to wash over the side.
He said he was pleased the sign had been installed.
"DoC has done a marvellous job and I have no doubt it will save lives."
The Mermaid Pools were listed on some online forums as one of the Tutukaka Coast's attractions but without reference to the danger, he said.
"I hope people consider the warnings."
There have been several incidents of injuries caused when the sea crashed over the ledge, smashing swimmers against rocks.
Last summer a man required an emergency airlift with a suspected broken leg. One woman said she and her 9-year-old stepson were among several people sucked under and pounded when three big waves crashed over the wall. She said she had feared for their lives.
Three years ago a toddler was washed out of the pool into the sea, but was grabbed by her father before being swept further out or onto the rocks.
The deep rock pool, known as the Mermaid pools or its traditional name Te Wai o te Taniwha, is near Rangitapu Pa, at the north end of Matapouri Bay.
DoC said the department had received no concerns from other public or emergency services about the pools' safety.
