Ahead of a trip to Western Australia, Pamela Wade worries about being swallowed up by the world's biggest fish.

David Attenborough nearly ruined my holiday. Looking forward to swimming with the whale sharks at Ningaloo Reef in Western Australia, before I left home I tuned in eagerly for his sea-focused episode of Life; and there, in the closing frames of the programme, was this fabulous fish.

It's the biggest in the world, yet - so I had already learned - it's a filter feeder of plankton, the smallest life in the ocean. All good: sure, that mouth is a metre wide, but it's a mail-slot, a narrow gap that I assumed was filled with mesh like a baleen whale's, thus presenting no risk of accidental swallowing to bulky creatures like me.

But here's the thing: Sir David's programme showed me for the first time ever a whale shark with its mouth open not just wide, but high. So high, in fact, that its gaping maw was almost round. Even rounder than me, I noted with horror, further shocked to hear him say that despite being a filter feeder, the whale shark possesses what seemed to me surely an excessive 300 rows of tiny teeth.

That would be tiny in relation to a fish that has been officially measured as reaching 16 metres in length, I reflected as the credits rolled. Suddenly WA's tropical north seemed somewhat less appealing.


And yet (spoiler alert!) here I am writing this, back home not only still in one piece, but completely unperforated by 300 rows of tiny holes.

This is neither a miracle nor a stroke of splendid good luck: it's simply normal, and precisely what Meg promised on the way to Draw Card's mooring for a day out on the reef.

"You couldn't possibly be swallowed," she said briskly; then added, slightly less reassuringly, "and if you were, you'd be spat right out again."

She was more interested in telling us about how the whale sharks come to the reef every year in April for the mass spawning of the coral that follows the full moon. The sharks stay in the area until July, making the reef the only place in Australia where it's possible to swim with what the brochures are unanimous in calling "gentle giants".

Ningaloo brings people flocking to Exmouth: here, unusually, 260km of fringing reef hugs the coast so that its hundreds of species of tropical fish, corals and molluscs are within snorkelling - even wading - distance of the beach.

It's the warm Leeuwin Current flowing down from the north that ensures this dazzling biodiversity, and on board Draw Card the water temperature gauge showed an astonishing 28 degrees.

"Oh, but we're still in the lagoon," said Elise. "You wait till we get outside the reef."

Enthusiastic, knowledgeable and with effortless authority, Elise ran an efficient operation. In no time, we were sorted into two groups, kitted out with snorkelling gear (including optically-enhanced masks) and wrist-to-ankle rash suits that were stunningly unflattering but essential to prevent snorkeller's sunburn on this gloriously sunny day.

The lagoon was that tropical turquoise that's impossible to look at without grinning; the off-shore breeze was balmy and full of dragonflies; and the sea was calm and clear. It was a perfect day to be out on the water.

Not, however, so great to be in it: when we arrived at our test snorkel spot inside the reef, a tiger shark glided past over the coral.

"Super cool!" enthused Elise. "Look at his beautiful stripes. We're so lucky!"

She brushed aside our anxiety: "If you're ever bothered by a dangerous shark, buy yourself a Lotto ticket, because it's so rare."

Always assuming you have an arm left to reach into your pocket for the money, I thought - but by then the shark had disappeared, and I was honour-bound to follow the Australians into the water.

The snorkelling was so brilliant that I immediately forgot about the tiger shark. More of a duck than a gannet where diving is concerned, I couldn't go deep enough to see the clams with their velvety purple lips and the sea cucumbers waving delicate blue feathers, but there were so many brightly-coloured fish, in such variety, in the top two metres above the corals that I couldn't keep count, spinning in circles to see new ones in every direction.

It was such a success that the whale sharks almost became icing on the cake; but we all cheered when the skipper announced that the spotter plane had located one, and we zipped outside the reef to go find it.

That was the end of the cruisy snorkelling. Although they look to be idling along, whale sharks are so big that they're actually motoring through the water, far too fast for us to swim alongside, and the only way to get a decent view is to be dropped off ahead of them, to watch them approach, pass by and disappear.

Elise, in sergeant-major mode, barked: "Group One! Gear ready! Back of the boat! Sit! Jump! Go, go go!" And we were in the water, heads under, looking to where she was pointing. At first, there were nothing but sunbeams shafting through the water; and then, literally out of the blue, we could see dots.

Like the Cheshire Cat, the shark's pale spots showed first, until it came close enough for its body to take shape: and what a body! At four metres, so amazingly big, the mouth wide, the head flat, gills pumping, the spots in a unique pattern of lines and swirls, tail sweeping in a leisurely arc. There was time to glimpse remora fish under its belly - and then it was gone, fading away into the blue again, and we bobbed on the surface, spitting out our snorkels so we could laugh in delight.

It was addictive, so when another shark was found not far away, Elise had no need to drill us, and we slipped eagerly into the water to see an even bigger shark, seven metres this time, loom up and glide past. This was a shark on a mission - places to go, things to do - and though the boat leap-frogged it twice, it was the briefest of encounters each time before it vanished; so we were thrilled that yet another was reported hanging around a nearby boat.

A four-metre youngster, it was gulping the bubbles from the engine-cooling water as it splashed into the sea, unbothered by the people hanging over the side, and the snorkellers in the water.

One of the sternest parts of our briefing had been to maintain a distance of three metres from the fish, to minimise interaction in the marine sanctuary, but no-one had told this shark. Finding the bubbles unsatisfying, it tried rubbing against the boat's hull before deciding to investigate Draw Card, and swim straight towards where I was scrabbling to swim backwards out of its way.

It was a heart-stopping moment, but not because I was afraid anymore of doing a Jonah: I was now such a whale shark fan that, despite its being a four-tonne monster, I wanted to protect it. It passed by so close I could have touched it - and it made my whole holiday.

Getting there: Air New Zealand flies direct to Perth. Drive north from Perth 1200km to Exmouth; or fly to Learmonth in less than two hours with Sky West or Qantas. Hire a car at the airport or take the shuttle bus 37km into Exmouth.

Where to stay: Bookings are essential in whale shark season. Try the new and comfortable beachfront Novotel.

Sharks: From April to July, several operations run whale shark tours; 3 Islands is thoroughly recommended for a professional and friendly operation. Eco cruises with swimming and snorkelling are offered during the rest of the year to see manta rays, dugongs, turtles and whales.

Other activities: Drift-snorkel at Turquoise Beach, take a glass-bottomed coral-viewing boat, cruise into the gorge at Yardie Creek, walk in the canyons of the Cape Range National Park. The Coral Coast is well worth exploring.

Further information: See westernaustralia.com and exmouthwa.com.au.