Installed MacOSX over HDD containing wallet I formatted and re installed MacOSX on the HDD that contained my wallet.dat. I have no backup of it.

Any ideas how I could possibly recover this file and my Bitcoins?
I have no experience and I think you would be mightily lucky if you succeed in recovering your wallet file, but I'd advise you to stop using that HDD (to avoid too much writes) and try googling for the problem and trying all the proposed solutions.