DLINK does not care about its customers!
I rang and they said it was my wiring. I then received the following email:
The Case ID that references your Technical Support mail is DAU3040078.
[Critical: Please do not change the subject line of your email when you reply. Leaving the subject line as it is will allow us to review your complete history and help us to better serve you.]
Sorry about the delay. The maximum throughput is based on theoretical transmission PHY rate. Actual data throughput will vary. Network conditions and environmental factors, including volume of traffic and network overhead, may lower actual data throughput rate. Interference from devices that emit electrical noise, such as vacuum cleaners and hair dryers, may adversely affect the performance of this product. This product may interfere with devices such as lighting systems that have a dimmer switch or a touch-sensitive on/off feature, short wave radios, or other Powerline devices that do not follow the HomePlug AV standard. Thank you.
Should you require further assistance with your D-Link products, please reply to this message, or call toll free at 1300 700 100 (AU) 0800 900900 (NZ). Technical support line is available 24 hours a day, every day.
Thank you for networking with D-Link.
In other words, bugger off, we don't care. The fact that I took the units to my inlaws house and tried the same tests and found the same result suggests that the units are either badly designed or faulty, but based on the response from DLINK, it sounds like they don't care so I'm returning the units back to the store where I bought them.