This is Australian slang
It is called a thunderbox because sometimes when you go to the loo you fart, and it sounds like thunder. I don't think I have to explain the box part.
"Well if you *must* know, it's my Thunderbox."
"May I see it?" he asked reverently.
"I don't see why not," said Apthorpe. "As a matter of fact I think it will interest you; it's pretty neat, a type they don't make any more. Too expensive, I suppose."
He went to the cupboard and dragged out the treasure, a brass bound, oak cube.
"It's a beautiful piece of work really."
He opened it, showing a mechanism of heavy cast-brass and patterned earthenware of solid Edwardian workmanship. On the inside of the lid was a plaque bearing the embossed title *Connolly's Chemical Closet*.
"I got it from a High Court Judge, the year they put drains into the Government buildings at Karonga. Gave him five pounds for it. I doubt if you could find one for twenty to-day. There's not the craftsmanship any more."