Possibly The Secret of Shelter Bay by Madeline Rose. It was published in 1982 by the Australian publisher Angus & Robertson. Summary retrieved from Google books:
The house and private beach entrusted to Andrew by a distant relative is also the home of the Merpeople and playground of the dolphins. When Andrew's father decides he must turn the area into a camping ground these two groups frighten away the developers and give the children treasure from a sunken wreck to solve Mr. Arden's financial problems and to preserve the privacy and safety of the bay.
A photograph of a couple of pages available on ebay shows kids exploring their new house and peering through a "big trap-door":
Tony and Lisa ran from room to room, exploring and exclaiming. They marvelled over the shells and peered with interest through the big trap-door.
The National Library of Australia notes the book is "For children".
The Charles Sturt University Library classifies it in the category "Nature conservation -- Juvenile fiction", echoing your description about an environmental theme.
Here is its record on isfdb, and its cover:
It seems difficult to find a digitized or paper copy, however it is available at the National Library of Australia (see previous link) and at a few other libraries in Australia and New Zealand, as you can see on Worldcat.
I found it by exploring the mermaids tag on isfdb, filtering by date, type, language, and then looking at the titles and other tags. Then I found the rest by looking for "The Secret of Shelter Bay" in various search engines.