A certain New Zealand Sci-Fi TV series that featured large snot-like creatures living under the sea ?

From my youth I recall seeing this program, I'd like to know who else does. It featured a young boy and girl who knew of the existance of such creatures - large snot-like creatures that lived under the sea, they roamed around in gl tunnels. Another detail I recall is that, besides themselves, there was only one other person, an old man, who knew of such creatures. I'd like to hear from anybody who recalls this program, and what the name of that program was. It was kind of spooky,