I’ve got a hammer in my hand when the cry goes up: “Orangutan in the camp.” Nine people down tools and grab their cameras and a chance to snap one of the critically endangered primates that we are here to help on our three-week stay in Indonesian Borneo. Rimba, a 17-year-old male orangutan, doesn’t disappoint: he circles the camp, going from tree to tree just a few metres above our heads for almost 30 minutes, hooting gruffly if anyone comes too close.

It’s a well-earned reward for our group, which has spent a week sawing, hammering, chiselling, drilling and painting in the 32C heat and almost 100% humidity of the jungle. The Pondok Ambung research post, operated by the UK-based Orangutan Foundation, is little more than a handful of single-storey wooden and concrete buildings. We got here by driving from the regional capital, Pangkalan Bun, then taking a four-hour boat ride from the port of Kumai, along the Buluh Kecil river in central Kalimantan (the Indonesian name for Borneo). We’re surrounded by towering ironwood trees, which form a canopy 50 metres above our heads, blocking out sunshine but trapping moisture and heat. Below it the thick tropical rainforest is alive with small things that crawl and fly.

There’s no mobile signal, let alone broadband. Our luxuries are packets of sweets and biscuits rationed to last the duration of our trip; a plate of sliced watermelon or oranges is greeted with delight. As Kalimantan is a dry state, the detox is physical as well as digital, and by the end of the first week we each feel we’ve sweated out more toxins than in a year of hot yoga.

The Orangutan Foundation runs its volunteer programme every summer, attracting adventurers who fly at their own expense and pay £800 to spend three weeks sleeping in basic accommodation and building infrastructure for the full-time Indonesian research staff, forest wardens and visiting students. Over the past 15 years, volunteers have built facilities including guard posts and boardwalks in Tanjung Puting national park, further south in Borneo, and the Lamandau wildlife reserve, a few hundred kilometres to the east.

I’m here because I want to do more for our beleaguered primate cousins than give money or avoid food and cosmetics containing palm oil. Naturally, we all want to see the orangutans. One of this year’s volunteers is back for a second year; others have given time elsewhere in animal welfare and environmental programmes.

Volunteer tourism has become big business, estimated at up to $2bn a year as tourists demand experiences beyond sightseeing. It’s also controversial thanks to the image of wealthy “gap yah” students and the scandal of orphanage tourism. Tourism Concern warns against joining volunteering placements that are little more than “expensive holidays.” Its guidelines lay out best practice for organisations, and its annual conference, in Croydon on 4 November (tourismconcern.org), helps potential volunteers look at their own motivations for volunteering and advises on questions to ask before signing up for a project.

My fellow participant Alec Watson, 45, who has volunteered on wildlife projects in Asia and China before, agrees it’s hard to find programmes with the high standards of the Orangutan Foundation: “Trying to find eco-trips where you can make an actual difference is not simple. A lot of them you just feel are cashing in on eco-tourism; here you’ve got a physical outcome at the end of it.”

The Orangutan Foundation’s programme flips the much-criticised colonial model on its head. Our boss in the rainforest is Jakarta-born Fembry “Arie” Arianto, the research and camp manager for Pondok Ambung, while our foreman on the building site is Kalimantan native Mat Jurie, leading a team of half a dozen camp staff who show remarkable patience with our basic DIY skills.

The work enables the Orangutan Foundation to study the wildlife of this protected area and to educate people about the risk of extinction faced by the apes and other animals. It’s a delicate mission: the palm oil industry is destroying animal habitats, but has raised the standard of living dramatically in a few decades.

For us volunteers, it’s not all work. There are several trips a short distance upriver to Camp Leakey, the pioneering primatology site established by Birutė Galdikas more than 50 years ago. Daily feeding time draws a crowd of orangutans, many of whom were released into the forest after being rescued from captivity as infants; it also attracts daytripping eco-tourists on boats from Pangkalan Bun ticking their bucket-lists or hoping for an Instagram hit.

Over our three weeks we get several visits from orangutans – either curious apes like Rimba or others just passing through as they forage for food and good nesting sites. Proboscis monkeys and macaques make their way noisily along the trees lining the river every day, and often just hang around in the trees opposite our jetty.

We’re also escorted on night walks to spot tarantulas, civets and tiny huge-eyed tarsier primates, and boat rides at dusk to look for saltwater crocodiles and their smaller relatives, false gharials. We become accustomed to the low-riding and wobbly motorised canoes that ferry everything along Kalimantan’s fluvial highways.

Every group of volunteers is different, but ours ranges from 18 to 60 years old, with eight women and four men when we begin. Despite the collegiate atmosphere, the basic conditions are hard to cope with: one volunteer takes a boat back to civilisation within a day of arrival; two more decide to return home after a week. It take all of us a few days to get used to the heat, and everyone is glad to escape from the mosquitoes when we leave.

The kitchen is two gas burners run by our cook and camp mum, Ibu Opit, who turns out a surprising variety of meals, although we can always rely on a base of sticky white rice. We take turns to assist Ibu in providing the national staple plus mie goreng (fried noodles), fried and stewed chicken, fried or barbecued river fish, prawn crackers, rice pancakes and banana fritters. After dinner, Arie talks about the challenges of conservation in Indonesia, and the Indonesian staff are keen to practise their English and share colourful Indonesian phrases. After three weeks we are all delighted with the smile on Arie’s face as he reveals that we’ve surpassed his plan for the programme: not only has the main building been rebuilt and extended, we’ve also repainted the outbuildings and his team has reroofed the accommodation block.

We could have seen wild orangutans on day trip to Camp Leakey, but we would have left Kalimantan as ignorant as we were on our arrival. While none of us is under the illusion that we’ve solved any of the knotty socio-economic problems that are driving orangutans to extinction, we have made a difference to people who are working to save them.

• A three-week stay with the Orangutan Foundation costs £800, including all meals, accommodation, outings and local transport. Next trip 23 July-13 August 2018. Garuda Indonesia flies to Iskandar on Borneo (via Jakarta) from £740 return