While there are some instances where lower voltage distribution lines and high voltage direct current (HV DC) transmission lines can be laid underground, there are significant costs to do so, as well as environmental, technical and operational limitations. In general, and for the transport of energy from large wind, solar, pumped hydro, and energy storage facilities, the transmission lines that we have today and need to build more of will be high voltage alternating current (HV AC). Placing AC transmission lines underground is estimated to be anywhere from three times to more than ten times the cost compared to placing HV AC lines overhead, and to take years longer to deliver. That additional cost would be paid for by all of us through our electricity bills.

Sometimes, energy generators or distribution businesses will choose to install underground lines – HV DC or lower-voltage lines – for a specific distance to improve their social licence, expedite their project’s completion, or where there are above-ground restrictions on doing so. This choice will require excavation and land disruption during construction and additional converter stations along the route, adding to land requirements and costs. For example, offshore wind project Star of the South plans to build a connection through Gippsland, which is to be built underground for a portion of the route. 

More information on the costs of under-grounding transmission lines can be found in the AEMO Transmission Cost Report 2021 (page 23), and in the AEMO VNI West Project Assessment Conclusions Report 2023 (page 105). 

Costs aside, underground transmission lines aren’t necessarily safer or more convenient. While they may ease some concerns around the look and feel of a place, they are not necessarily better for the environment or landholders. There are significant land disturbance issues associated with the initial trenching works, and accessing underground lines for maintenance is more invasive as soil including any crops on top of the line must be dug up if there are faults. In contrast, overhead lines accommodate the majority of agricultural land uses.