The third location used in Cobwebs in the Dark is Kaipaki, near Cambridge on New Zealand's North Island.
This is where Natasha was going when she took off on her adventure, and the house she went to looked something like the one in this picture.
I don't think it's giving too much away to say that she had to catch a bus to Cambridge, and then a taxi to Kaipaki from there, which was fairly daunting for a thirteen year old on her own in a strange country.
Of course her trip brought problems of its own, but you'll need to read the book to find out about those!