Some power lines will need to be moved, towers will need to be replaced and some towers will be raised with possible visual effects, he said.
Gardner-Hopkins said Transpower was confident that effects on the national grid will be minimised, saying NZ Transport Agency will pay for the work.
Among the lines affected are the Henderson-Otahuhu line, which supplies 70 per cent of the transmission capacity across Auckland.
"Without this line, the existing peak load in Auckland and North Auckland could not be supplied," Gardner-Hopkins sad.
The project will also affect the Penrose-Mt Roskill line, which supplies one of three substations sending power to central Auckland, and the Mangere-Roskill line, which feeds the same substation and also acts as a back-up to the central city network.
Transpower's substation at Southdown, which supplies Auckland's electric rail network, is also affected.
Spark is also affected by the huge road project along the northern edge of the Mangere Inlet, telling the board of inquiry its Hamlin Hill and Otahuhu cellular sites need to be substantially reconfigured and moved respectively.
Spark is not opposed to the project, its counsel Daniel Minhinnick said, provided towers were moved or reconfigured properly, saying it was negotiating with NZTA.
"These negotiations have not progressed to the point where Spark is comfortable that it can withdraw from the hearing process, which is disappointing from Spark's perspective," Minhinnick said.
The board of inquiry, chaired by retired High Court Judge Dr John Priestley, is set to run until August 25.
The board will release a draft decision on October 9. Following comments from submitters on minor or technical matters, the board is due to make a final decision on November 22.
Construction is expected to begin late next year and be completed by 2025.
