How can I get my uploads working again?

Recently, all types of uploads have stopped working with my internet connection. I cannot upload anything to deviantART or facebook, and I can’t even send emails with my mail application.

I have an iMac, a macbook and a windows laptop all on the same network sharing the internet connection, and they all have the same problem.

When I noticed the problem I ran a speed test at speedtest.net. The download section of the test worked perfectly, but the upload section wouldn’t even start.

My ISP is iiNet, and I live in Australia.

Anyone know of ways I can diagnose the problem and get it fixed? Thanks very much in advance.