Solved Girl in Sydney finds out she is a witch, using Fibonacci sequence to make magic. Her mother is out of the picture and not of sound mind, there's a door leading to somewhere that snows. [s]

[Sharnie]

There are at least three books in the series, but that's pretty much all I remember.
The girl is a witch, she uses the Fibonacci number sequence to make magic. There is a door in one of the later books connecting her house to a place that snows (New york stands out for some reason).
What I do recall is her meeting with her mother at one point and their being magic butterflies flying around them.

[Michelle Mulford]

The Magic or Madness series, by Justine Larbalestier. These are sad, but really good.

[Sharnie]

Aye! Wonderful, thank you very much