Safari's "AutoFill Form" (Cmd-Shift-A) function is useful for quickly entering name and address details in a web page from your "own" address card in Address Book, but it fails to complete what we call the "postcode" in Australia. Probably because it's looking for a field labelled "zip code", I assumed, and Australian web forms have this labelled "post code" or "postcode". So I wondered if this was configurable.
I found the solution it in Contents/Resources/English.lproj/ABAutoCompleteMappings.plist within the Safari.app package. To open this in the Finder, right-click (or Ctrl-click) on Safari.app in the Applications folder, select "Show Package Contents", then navigate to Contents -> Resources -> English.lproj. Make a backup of ABAutoCompleteMappings.plist (Cmd-D does this nicely), then open ABAutoCompleteMappings.plist in Property List Editor (if you have Developer Tools installed) or any text editor.
If you are using a text editor, you will find the following starting at or around line 134:
<dict>
<key>ABProperty</key>
<string>Address</string>
<key>ABKey</key>
<string>ZIP</string>
<key>FieldLabels</key>
<array>
<string>zip</string>
<string>zipcode</string>
<string>zip code</string>
<string>postalcode</string>
<string>postal code</string>
</array>
</dict>
So now we know that Safari responds to a field labelled "zip" or variants (for the USA obviously) or "postalcode" with optional space (I guess for the UK and/or Canada).
...
<string>postal code</string>
<string>postcode</string>
<string>post code</string>
...
If using Property List Editor, add the extra entries to Root-> Mappings-> 11-> FieldLabels.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070527063904285