In "Environmental Load Reduction Effects of Ubiquitous Broadband Services" (2007) NTT researchers compared the CO2 emissions from purchasing music in a physical store with downloading it using a fibre to the home broadband network and a wireless network. They concluded that the most energy efficient is the wireless network (31 kg CO2e/year), followed by fibre to the home (108 kg CO2e/year) and lastly conventional physical delivery (201 kg CO2e/year). I get my Green IT students to examine this analysis.
This is relevant to the discussion of the NBN in Australia, which is a fibre to the home system. A wider analysis might show better savings for the fibre system, as there are energy savings tasks which could be performed with it, such as high resolution video replacing travel, which are not possible using lower speed wireless.
Techniques could also be used to reduce the power consumption of the NBN. The MaxLinear MxL261 digital cable front end chip, for example, has a low power mode which can be used when less bandwidth is required and to save battery power.