The link crosses Cook Strait, between the two islands, using submarine power cables laid along the sea floor.

The line turns east and then south-east around Port Underwood, before crossing to Fighting Bay on the coast, where the South Island cable terminal is located.

At this physical location, the lines connects to three undersea cables taking electricity underneath Cook Strait.

From Oteranga Bay, the land-based North Island transmission line travels northeast through Mākara just west of Johnsonville.

They are constructed with a compacted multi-strand copper conductor as a central core, with mass-impregnated paper insulation surrounded by a lead sheath.

Anyone found anchoring or fishing in the area is liable for fines up to $100,000 and forfeiture of their vessel – more if a cable is subsequently damaged.

The longest span is 1119 m, near Port Underwood, close to the Fighting Bay cable terminal station.

Following the DC Hybrid Link project, the line was rated to carry 2000 amperes continuously on each pole, at an operating voltage of 350 kV HVDC.

The cells are surrounded by selected and graded stones and geotextile layers to allow seawater ingress, but prevent the buildup of silt.

Each electrode arm is a 40 mm mild steel rod buried in a coke bed of around 0.26 m2 cross sectional area, in a 1.5 m deep trench.

Maintenance outages are planned well in advance to minimise the effects – they are usually carried out in summer when national electricity demand is at its lowest, and on only one pole at a time, with the other pole remaining in operation providing half of the full two-pole capacity, using the earth electrodes providing a path for return current through the ground.

Notable faults and outages on the HVDC Inter-Island link: The initial vision for electricity transmission between the South and North Islands was developed by Bill Latta, the Chief Engineer of the State Hydroelectric Department.

In 1956, the Government appointed BICC to undertake detailed investigations of the practicality and cost of a Cook Strait cable crossing.

[15] In parallel with the technical investigations for cables underneath Cook Strait, the Minister responsible for the State Hydroelectric Department appointed a committee of key stakeholders to report on the options for power supply to New Zealand as a whole, not just the North Island.

Recommendations were also received from the Swedish company ASEA (today part of the ABB), about the technical aspects of the HVDC converter stations.

The unique planning considerations for the overall proposal included:[12] In 1958, BICC laid two 0.8 km trial lengths of cable off Oteranga Bay in Cook Strait to demonstrate their ability to resist the abrasion, bending and vibration caused by conditions on the seabed.

[12] In the period 1958 to 1960, some differing views were offered to Government about the most appropriate power developments for the country as a whole, and there were reservations about the risks involved in the planned Cook Strait cable crossing.

[23] The terminal stations at each end of HVDC link used large mercury-arc rectifiers and inverters – 1960s technology – to convert between AC and DC.

[24] Until it was upgraded in 1993, the HVDC Inter-Island link had normal operating voltages of ±250 kV, and a maximum power transmission capacity of about 600 MW.

In 1976, the control system of the original scheme was modified to allow power to be sent in the reverse direction, from Haywards to Benmore.

However, in December 2007, Transpower announced that one-half of the capacity of Pole 1 would be returned to "warm standby" service before the winter of 2008 in order to meet the demand for power in the North Island if needed.

[29] On 13 March 2008, Transpower announced that work had been completed to restore 50% of the capacity of Pole 1 to service at times when the demand for power on the North Island peaked.

The energy transfer on Pole 1 was strictly limited to the northbound direction, to reduce the stress and strain on the aging converter system.

[24] On 1 August 2012, Transpower decommissioned the remaining half of the Pole 1 mercury arc valve converter stations at Benmore and Haywards, after 47 years in service.

The Inter Island link at the time was the last HVDC system in the world with mercury arc valve converters in operational service.

Site works on the $672 million project were formally commenced on 19 April 2010, when Minister of Energy Gerry Brownlee turned the first sod.

The 16 kV generator busbars in the power station were the point of connection between the HVDC link and the South Island grid.

Following Transpower's decommissioning of the original Pole 1 equipment, there was no longer any direct connection between the generator 16 kV busbars and the HVDC link, and the limited capacity of the Benmore interconnecting transformers would have constrained the maximum output of the station.

[36] [37] In 2024, the Commerce Commission approved a proposal from Transpower to upgrade the capacity of the HVDC link by adding a fourth Cook Strait cable and additional reactive support at Haywards.

The upgrade was approved as a contingency, where the timing would depend upon triggers such as the exit of the aluminium smelter from Tiwai Point, developments in the load and generation forecasts, and Transpower demonstrating a net positive market benefit from the investment.