The tau are a relatively new race in the WK40K, the Necron are an ancient one. According to the 6th edition wargame rule book allies table, Tau and Necron forces are 'Allies of Convenience'.
Necron fluff has supposedly changed (in sixth edition) so they don't have to be merciless killing machines.
Is there anywhere in cannon that shows they've allied or opposed each other? Either in the snippets of fluff or the various novel for the WH40K universe?
When I looked on Google I found forum posts saying may had happened, and was mentioned in a codex somewhere.