What began as a direct ferry route to shift freight quickly became a part of the quintessential Kiwi summer.
The company has big plans for new terminals on both sides of the strait, and two new hybrid-electric ferries.
On Sunday, it will be setting up shop in a shipping container in Circa Plaza, near Te Papa, to show off its history and its plans for the future.
The first Interislander ferry was the Aramoana.
The Aramoana first set sail in August 1962, and replaced the sole passenger service, the Tamahine, which had been running for 40 years.
Built in Scotland, the Aramoana weighed 4000 tonnes and her name meant "pathway across the sea".
Interislander executive general manager Walter Rushbrook said the arrival of a roll-on, roll-off ferry changed the way people and freight moved around the country.
Prior to the Interislander, the only option was a coastal ship, loaded with people via gangways and freight via cranes - a time-consuming process, he said.
"It was a bit of a game-changer, because you could take both railway wagons as well as road vehicles onto the ship."
A new rail connection between Picton and Christchurch meant the main trunk line now ran the length of the country, and the ferry link meant goods loaded in Auckland could travel from Auckland to Christchurch in three days, and arrive in the same wagons.
The Aramoana made 12 crossings a week. These days, Interislander offers seven return sailings a day.
A fast ferry service was introduced in 1994, named "the Lynx" - a play on the way the service "links" the two islands - which cut the crossing time to one-and-a-half hours.
It was not, however, renowned for its comfort, earning the nickname the Vomit Comet.
But by 2000, there were growing concerns about the impact of fast ferries on the shorelines in the Wellington harbour and the Marlborough Sounds.
Rising fuel prices and mechanical issues made the Lynx less and less economic, and it was scrapped in 2005.
Plenty of New Zealanders remember their journeys on the Interislander - Rushbrook was only six when he boarded for the first time.
"We drove all the way down from Rotorua to catch the ferry," he said. "It was my first time on a big ship. So as any six-year-old would be, I was just wide-eyed and just blown away."
Last month, a long-lost life ring was returned two decades after it was taken from the Arahura, along with a note.
"Hello, this is something I have wanted to do for some time - return this life ring to its rightful owner.
"Took possession of this life ring on a ferry crossing in the early 2000s and I'm sorry.
"Please restore this to the owner on my behalf."
Rushbrook said the ring was now on display in the Wellington terminal, behind the check-in counter.
Interislander owner KiwiRail signed a $550 million contract with Hyundai for two new ferries to replace their ageing fleet, Rushbrook said.
It would nearly double passenger capacity, and reduce emissions by forty per cent.
Two new terminals are also in the works, in Wellington and Picton.
Although it's a time of change for the company, for many New Zealanders an old song by The Warratahs will always remind them of the journey: "Cruising on the Interislander, sailing to the other side."
The band performed aboard the Kaitaki last weekend as part of the anniversary celebrations, and 30 years on The Warratahs are still asked to sing Sailing to the Other Side (The InterIslander) at their shows.
