This children's television show was broadcast in Australia in the 1980s, most likely on the ABC, and featured children befriending a creature that might have looked a little like E.T. The children and the creature would slide down an organic-looking brown/green tube to get to the creature's lair, which was fairly small and may also have been the base of operations for the children and the creature - it may have had computers in it. I'm afraid that is the only clear memory I have of the show. I think the children were the main characters, along with the creature, but I can't remember anything that they did.
I wonder if you are thinking of the 1981 tv adaptation of "Under the Mountain"? The children, Rachel & Theo, don't befriend any alien creatures, rather they are trying to stop them from taking over New Zealand (and the rest of Earth).
There is a clip here that shows the Wilberforce's lair. https://www.nzonscreen.com/title/under-the-mountain-episode-four-1981 They are giant slugs so their lair is very organic, and the children slide down into it.
The tv program (and 2009 film) are based on the book of the same title by Maurice Gee.
Maybe this is too obvious, but this made me think of Fraggle Rock. It is from the 1980s, was broadcast in Australia, and features an intro with puppet traveling down a tunnel to their lair.
There were also interactions between the Fraggle world and real humans, through the adventures of Traveling Matt, e.g.