Movie about a group of kids on a treasure hunt to Australia?

[calenture]

I think it came out in the early 90s or late 80s. There's a group of kids, they find a letter that talks about a treasure on the "other side of the world", and they set off to find it. One of them initially thinks the treasure is in China but then they realize it's actually somewhere in Australia. All I remember is that when they get there, the "treasure" is a stamp collection and possibly a baseball glove as well?

For some reason I always thought it was The Goonies 2, but apparently they never made a sequel.