Storm clouds were gathering as we sped up the Kinabatangan River. Branches hung like tangled ropes from the river banks. ‘Look!’ said our American guide, pulling out a camera with an enormous lens. ‘A rhinoceros!’ Ten pairs of eyes darted towards the muddy water, suddenly fearful. Faced with a charging rhino, might our little boat capsize?

‘I think he means a rhinoceros hornbill,’ whispered an Australian grandmother, pointing to an orange-beaked bird clapping its wings above us. ‘Not sure the rhinoceros exists in Borneo.’ Hearing blunted by the clamour of the boat’s engine, her compatriot, a baseball-capped Sydney hotshot, scanned the river for four-legged beasts before reaching for his binoculars. ‘Too right, mate,’ he said. ‘If it’s in the water it’s probably a hippo. But don’t they all live in Africa?’

Such muddles are all part of the fun for jungle first-timers. Our expedition deep into Borneo’s densely forested interior is just one of many adventures laid on by Orion II, a 50-cabin Australian cruise ship with an unusual USP. Along with 70-odd Sydney professionals (mostly in their fifties and sixties), I have signed up for an extraordinary voyage focusing on endangered animals, conservation and second world war history. Our journey will begin — and end — in Kota Kinabulu, the provincial capital of Sabah in Malaysian Borneo.

Most mornings, after a slap-up breakfast on deck, we clamber onto Zodiacs driven by handsome, uniformed Filipinos, arriving in pungent harbours filled with cargo ships and cranes. Although there’s often an awful lot of rubbish floating along the coast, it’s romantic in a gritty sort of way. Waiting for us on the quayside is a rolled-out carpet, a tent and a line-up of shuttle buses. It’s a bit like being royal. Everybody waves, even the workmen lifting containers in the scorching heat. With us from the ship is a clutch of experts, walking encylopaedias on birds, beasts and biology, who urge us to look at things that ‘move, fly, slither and crawl’.

At the Labuk Bay sanctuary, a patch of forest surrounded by palm oil plantations, we descend on the monkeys in a paparazzi mob, but the animals don’t seem to mind. The proboscis monkey, an endangered species with a large, dangly nose, pot belly and vigorous libido, poses willingly, swinging onto our boarded walkway with loud, enthusiastic honks. Later that day we visit the hugely popular Sepilok sanctuary. Here I discover that orangutans share 96.4 per cent of our DNA and have an impressive line in yoga poses. Adult males can weigh more than 260lbs and are four times stronger than their human equivalent.

On a walk across Pulau Tiga, one of a chain of three uninhabited islands that hosted the TV series Survivor, I learn that a single hectare of tropical rainforest can have 220 species of tree. Our guide is an impassioned conservationist. ‘Anything that’s here came on the wind or was attached to a bird’s foot,’ he says. As we plunge deeper into the forest, the cicadas go into overdrive, ringing out like car alarms to warn of our arrival. Under a canopy of trees, the sound of the surf rolling in the distance, we float half-naked in a bubbling mud volcano, happy as toddlers in a sandpit.

Other moments are more profound. At Sandakan Memorial Park, I learn how more than 2,000 Allied prisoners were marched to their death in the jungle by their unimaginably barbaric Japanese captors (just six, all Australian, escaped). Later we visit the airy, hilltop home of Agnes Keith, an American writer who kept a diary, hidden in tins, of her imprisonment under the Japanese (she survived the ordeal and her memoirs, Three Came Home, were made into a Hollywood film starring Claudette Colbert).

In the rush of activity it’s easy to forget things. But every evening we gather, cocktails in hand, for a recap and slideshow of the day’s activities, masterminded by the ship’s expedition leader. During dinner (good, occasionally excellent, food), the following day’s programme is slipped under the doors of our cabins.

Expedition cruises aren’t, of course, for everyone. Occasionally I felt lazy and rebellious. I longed for big ship luxuries — a pool, broader decks, a choice of places to eat — but I loved broadening my mind without having to plan or organise anything. Another perk was the social life. Australians are the ideal nation to sail with. Friendly and open-minded, they made me laugh, joined me on the dance floor and called me ‘the English rose’. (Sadly, quickly replaced by ‘You are very red!’).

Many things will stay with me — the heat, the snorkelling off Mataking Island, the orangutans with their doleful eyes and shaggy coats, the Rungus tribe with their beautiful beaded necklaces — but perhaps it’s the cloud formations and sunsets that will linger longest. One evening the ship’s captain turned off the lights on the bridge so that I could see the stars. Above me the milky way arched across a glittering night sky.

It was a perfect cruise ship moment.

Ten nights from £4,890pp, cruise only (020 7399 7620; www.orionexpeditions.com). 

This article first appeared in the print edition of The Spectator magazine, dated