Sue Baxalle has a close encounter of the whale-shark kind at Ningaloo Reef in the far north of Western Australia.
Heads down now!" calls Tayla, our guide. Eager to have my third up-close sighting of one of the world's biggest fish - and hoping to be able to keep up better this time - I do as she says, wondering, as with my previous swims, from which direction the whale shark will swim past us.
Seconds later, I am looking straight at a metre-wide mouth, heading directly towards me.
Though I know the plankton- and krill-eater poses no real danger, its sheer size is overwhelming.
And the warnings of the guides on our Charter 1 vessel, Concorde, are fresh in my mind: keep to the side of the whale shark, about 3m clear and 4m from the powerful tail.
Somehow, nobody had told this shark the rules and he (almost certainly a male, as the Ningaloo Reef is a haunt for migrating juvenile males between 4m and 12m in length) changes course.
I quickly forget all about filming the creature and concentrate on the fastest swim of my life.
The main reason we had been warned to keep clear, not to try to touch the whale shark or get in its way, was so it would not dive and spoil the sighting for others.
Whale shark spotting is the main tourist attraction from March to July for visitors to Exmouth, on the northern tip of Western Australia. There are 15 whale spotting operators licensed by the Department of Parks and Wildlife and they all leave from the Tantabiddi boat ramp, about 36km drive from Exmouth.
My outing begins when I am picked up by guides Georgia and Tom in a minibus on a tour of the town's accommodation to collect passengers. With Tom at the wheel, Georgia chats about what we can expect of the day and information about the sea creatures of Ningaloo - as well as the main focus of whale sharks, groups often see turtles, manta rays and dugongs.
She also points out landmarks such as the Harold E. Holt naval communication station and the Vlamingh Head Lighthouse. Tom takes a detour to go up the hill to the lighthouse and give us a view of the Ningaloo region. Ningaloo - which means high land jutting out in the sea - was given World Heritage status in 2011.
The Ningaloo Marine Park protects the 260km fringing reef and the Cape Range National Park, 50ha of rugged canyons and ridges along the peninsula.
Georgia tells us the reason the reef is home to a multitude of colourful tropical fishes is the meeting of two opposing currents, the Leeuwin current and the Ningaloo current, creating an eddy which brings up nutrients.
In March-April the coral spawns. Krill and other tiny marine creatures swarm on the reef to feed on the spawn, attracting larger predators, including the stars of our excursion, the whale sharks.
Georgia explains the gentle giants of the sea can dive to about 3000m, and do not need to surface, doing so only for thermal regulation. "It gets pretty cold down there."
Little is known about the lifestyles of the whale shark and its mating patterns. Georgia says one female, harpooned off Taiwan, had 300 embryos inside - all at different stages of gestation. But, when tested, they were all from the same male.
Ningaloo's whale shark adventure operators log all their sightings and passenger swims. Photographs of the creatures' "fingerprints", a pattern of spots around their gill area, are sent to researchers.
As for predators, the whale shark's size makes it relatively immune to all but mankind. Georgia tells us that fins are worth US$1500 ($2137) in Asia and, though apparently no good for shark fin soup, they are considered a trophy for shop displays.
Along Ningaloo Reef, the whale sharks are up to 12m long, but the largest seen was, reportedly, about 16m, Georgia says.
"But don't worry, their oesophagus is only the size of a fist, so they're not going to eat you, just krill."
At Tantabiddi we board the tender that takes us out to Concorde, Charter 1's 14.5m vessel, captained today by Sarah Ellis, who owns the company with partner Matt Oakley. As we head out to our first stop - a snorkelling spot in the lagoon - we are fitted for wetsuits, fins, masks and snorkels. Prescription lens masks are available, too. The team give us the run-down on whale shark rules, and then it's into the pleasantly warm Indian Ocean to see myriad fish and corals.
Some of our group are lucky enough to spot a turtle - I always seem to be in the wrong place.
Back on board there's morning tea and then we head off to hunt the whale sharks.
Spotter planes are circling overhead to make the task easier for Sarah and to ensure we get what we came for.
The boats and planes are in constant contact, and it is the first boat to the scene that has first dibs on the giant fish.
As there is a limit of 10 swimmers at a time with the whale sharks, we are divided into two groups, one going with Tayla, one with Georgia.
I'm in group one, and join the others, lined up on the rear deck of the boat by the lowered swimming platform, ready to plunge at Tayla's word. As we jump in and move towards our first encounter with the biggest fish in the world, I realise that I'm not scared, the adrenalin flowing though me is driving me on.
Our first one is young, about 5m, the experts later tell us. Although they say he was swimming slowly, he was too fast for me. Nevertheless, I'm in awe of this graceful spotty creature and keen for a second swim.
Once back aboard, we travel on a bit so group two can be deposited alongside his path.
Meanwhile, the spotter plane has been searching for another gentle giant to swim with, and off we go, again and again.
My third dive is the one in which Wally the whale shark takes it into his rather large head to get closer to me, but once I'm back on board Concorde and my nerves settle, I'm keen to give it another go.
A similar thing happens but we are prepared and able to watch Wally silently glide past. The view, so close, is unbeatable.
I ask Sarah how far out we have gone - it is about 10 nautical miles west. She says there is banter among the whale shark contractors about who will be the first to Madagascar.
The monsters of the deep are not our only wildlife encounters. We have a visit from a very friendly petrel and will spot a pygmy blue whale on our way home.
After a couple more swims we leave the whale sharks to their own devices and head off to another choice snorkelling locale, having lunch on the way.
As well as snorkelling, we can use the vessel's kayaks and paddleboards.
Before we get there, however, we are joined by an enormous pod of spinner dolphins - hundreds of them racing and playing with our boat, putting on a great show.
Too soon it is time to head back to Tantabiddi - enjoying a glass of celebratory bubbles on the way. The day was exhilarating - and it was tiring trying to keep up with the beasts.
It is possible to go just as an observer, but a plunge in the ocean with these majestic creatures is definitely recommended - though it would be a good idea to practise swimming to gain strength if you're planning a whale shark trip.
Getting there: Qantas flies twice daily (excluding Fridays and Saturdays) from Auckland to Exmouth, via Perth.
The writer travelled as a guest of Tourism Western Australia and Qantas.