Site http://scentaliciousbytanja.com.au/ that I'm working on crashed only showing:
Parse error: syntax error, unexpected ':' in /home/......./public_html/wp-content/themes/atahualpa/index.php on line 2
I am sure this is a hack of some sort, easily fixed but thought I should bring it to your attention.

Fixed by replacing the wp-content/themes/atahualpa/index.php with the original by FTP.

Any one with ideas on how this happened?

ooops spoke too soon all user ID's and passwords have been changed

Any one with ideas on how this happened?

someone cracked your password (did you know that 'password' is the #1 password people use?)
someone hacked the server you are on
someone hacked your computer and installed a keylogger
someone use a vunerability in some old plugin you are using
someone discovered a new security exploit and used it on you.

how's that for a few possibilities? Seriously, unless you have access to ALL the logs from your server for the last couple weeks and want to spend a lot of money on someone doing a forensic investigation, ther is probally no way to know

I have found an amazing plugin called Wordfence it handles a huge amount of security issues and sends you emails if the site gets tampered with, I have more than 40 wordpress sites now and run it on all, also lets me know when things need an upgrade.