Night of the Red Hunter
You're right, it is an obscure show. But there have been others looking for it in the past :-) Anyway, here's a synopsis:
Night of the Red Hunter is a 1989 mystery series aired on Television
New Zealand about two runaway children named Peter and Maggie. The two
bump into each other in Piper Valley, a small countryside town. A crazy
man chases them into a graveyard where they choose to hide.
They stumble upon the "treasure":
The kids accidentally trigger a secret opening in the graveyard and
fall. They find themselves in a room filled with intense light.
Looking around, they discover strange equipment and millions of
dollars in gold artifacts. But before they can explore further, they
How these kids meet the aliens:
The children wake up the next day with no memory of the previous night
in the farmhouse of the Pipers, a seemingly friendly yet somehow
sinister family. They turn out to be a race of benevolent aliens,
stranded on Earth until they get their ship repaired.
More about the mysterious cache of gold:
The aliens are allergic to the gold which they require to power their
ship, mentioning in passing how the Earth is poisoned by the toxic
effects of our own fuels.
A brief mention of the comet you brought up in your post:
Meantime, the comet Nimrod is about to pass close to Earth, but Nimrod
is really a planet-killing weapon with the power of a thousand nuclear
weapons, and the Earth is doomed unless the scrappy runaways cooperate
with each other and a slightly older alien child.
~ Edited from tv.com and Time Capsule (by J. P. Harris)
Find out more about the show and watch a few clips here.