Each year the whales migrate to the warm, safe waters of Tonga to mate and give birth. They usually stay in the area from July to October, while the mothers feed their babies a staggering 100 litres of milk a day in preparation for the 8000km swim back to Antarctica.
Their months in the South Pacific offer a rare and incredible chance to have an encounter with the mammals, and I feel very lucky to be getting the opportunity.
In his former life, Allan worked in advertising in Auckland. Leathery-tanned, bearded and with a delightfully cheeky smile, it's hard to imagine him in that world, when he is so utterly at home in this one.
He and wife Lyn moved from New Zealand to Tonga 20 years ago. Now - with Kirsty, who joins them as a guide during the season - they run Whale Watch Vava'u, and the incredible Mounu Island Resort, a paradise only 30 minutes from the mainland by boat.
The island of native bush and white sand surrounded by bright clear water is all about peace and privacy - the mantra of Mounu - also known as barefoot island, Allan tells me.
It has only four solar-powered fales (houses), spread evenly around the circumference, giving each guest their own private section of beach and meaning no more than eight guests stay at a time.
People come from around the world to experience the resort, and swim with the whales, Allan says.
The night before our whale encounter, over freshly toasted coconut shavings, I meet the other two guests on the island who will be on the boat tomorrow - an Australian couple who return to Mounu for a week each year to swim with the whales. Every day is different they tell me, and every encounter special.
The next morning a small group of us get ready to head out with guide Kirsty and skipper Nati. There's excitement in the air.
I've been warned by Allan and Kirsty to have realistic expectations. We will see whales but whether we swim with them is up the mammals - you can't predict wildlife and they usually tell people to allow three days for an encounter.
I'm booked for only two, so have high hopes luck will be on my side today.
On board, Kirsty tells us about the whales and their habits, and that we're in safe hands. As baleen whales, they don't have teeth and instead use their baleen plates for filtering food from water.
For safety there are only four people in the water with a guide at any time, swimming is only done with whales who want to be swum with and touching is definitely not encouraged.
On the water we're all searching for signs of whales, specifically water from their blowholes, indicating they're nearby.
We finally see one and skipper Nati directs the boat towards it to see if it wants to engage. Knowing whether they do is not easily explained, but comes from years of experience and understanding them, says Kirsty.
After establishing they are happy for us to be there, we quickly don snorkels and flippers and get into the water, sliding off the back of the boat and swimming just under the surface.
Under the water, I'm not ready for what I see. They're so close, I can see the eyes, see the sucker fish feeding off the skin of the mother, her long white markings and the calf turns to look at us before gliding off. It's serene and peaceful and quiet.
It doesn't feel real - more like you're watching a movie of yourself dwarfed by these mammals that grow to be about 16m long and weigh 27-45 tonnes. Its an experience you never think you'll have; a world you've not been let into before.
At the surface Kirsty grins at me - "Abby, you've just had your first whale encounter!" I beam back.
We find more whales - another mother and calf, with a male escort deeper below them, and watch the adult whales playfully breaching on the surface. Sometimes swimmers hear and feel the vibration of the whales singing to each other, says Kirsty.
Back at the resort, she tells me why she loves what she does. "It's such a unique experience; such a special buzz."
Though they are often confronted by the argument that they are harassing the whales, speaking to Allan it's clear no one cares more about these mammals or has their best interests and protection at heart. We are entering their environment and everything is on their terms.
What he is offering he says, is an experience, "an interaction between you and the whale".
CHECKLIST
Getting there: Air New Zealand operates five direct flights a week from Auckland to Nuku'alofa.
Further information: Mounu Island Resort is on a 2.6ha coral atoll in the Vava'u group of islands. Whale Watching Vava'u is also run from the island by the owners, who were the first licensed operators in Tonga.
Abby Gillies was a guest of Mounu Island Resort and went swimming with Whale Watch Vava'u.
