Is it a small office that shares a single router for web access?  Or is it a larger place which may have a proxy and their own DNS servers?

Generally speaking, if its the latter it is most likely a problem in house.

A quick test (ensuring you are not breaking any I.T rules) bring up a command prompt on your PC (can be done by typing in cmd in the run box)

At the command prompt type in ping www.aussiepinball.com.au

You should see it bring up immediately the words " reply from " then a heap of numbers....

If you get nothing or a "request timed out error" then its either an issue with your I.T systems internally, or your ISP's DNS servers.