And in doing so, he and a dozen other foreign volunteers, who wear rubber fishermen's waders and clothes-pegs on their noses to block the gut- churning stench, are taking on a challenge that even Mahatma Gandhi failed: to revolutionise the idea, strongly ingrained in the Hindu caste hierarchy, that only the untouchables are fit for getting rid of human waste.
The Madras bureaucrat has kept Mr Hendry's passport in his desk for several months now. The Australian is trapped in a legal limbo. He cannot leave India or even Madras, without his passport. But that suits Mr Hendry; he is happiest mucking out the city's ghastly latrines and open sewers.
A more revolting job is hard to imagine. Public toilets anywhere in India are seldom cleaned. Madras is also plagued by severe water shortages so the city's poor have no choice but to squat alongside the pigs and wild dogs in the open sewers. Riding a train early in the morning in India, it is not unusual to see hundreds of people defecating quite naturally along the railway tracks, all with their precious jars of water to wash themselves.
Like most Asians and Middle Easterners, the Indians find the use of toilet paper disgusting. One Indian classical dancer famously complained at the Albert Hall that her concentration had faltered because she had been distracted by the thought of all the unwashed bums in her audience.
The Australians do not aim to upset the ancient Hindu caste system. They simply want to improve the sanitation for Madras's millions of slum- dwellers. Their motto is: 'Shit kills', and they know this at first hand, having been stricken by the usual diseases preying on India's poor: typhoid, dysentery and boils.
When the Australians descend on a public toilet, noses firmly pegged, they are often met by hostility. The slum people try to wrestle away their scrub brushes and mops. 'Stop it, stop it] Go away] You're shaming us,' they cry. Kevin McKay's reply is: 'OK, give us a rest and take over yourselves.'
Mr McKay, 33, is a gifted illustrator sought by Australia's Prime Minister to design his official Christmas cards. In Australia, he could be living comfortably, with acclaim, instead of spending his days in torrential monsoon rains up to his knees in excrement. So why does he do it? He and the other Australians and New Zealanders are Christians, but not aggressively so. They are not seeking Hindu converts.
The idea of helping Madras's poor came from Kevin's father, David McKay, a tempestuous one-time Methodist preacher who began to see organised religion as an axe that cleaved men apart. He took Kevin and the rest of his family out of the city to live a spartan existence in tents along a river in the Australian outback. The family worked among Aboriginals cursed by unemployment, illness and drunkenness. Some joined the McKays, attracted by the father's iconoclastic good deeds. Much of the family's time was spent trying to raise money. They painted the homes of wealthy Sydneyites, until it all seemed pointless. 'We thought that if we really wanted to serve, we'd go where we were most needed,' said Kevin McKay. Inspiration came from Richard Attenborough's film, Gandhi. 'Most of us had never heard of Gandhi until we saw the movie,' said Mr Hendry, a former aviation mechanic born in Scotland. So they chose India.
The Australians ended up in Triplicane, a neighbourhood on the edge of Madras's most vile slum. 'We just started clearing the street in front of our house,' said Mr Hendry. At the same time the group, who considered themselves 'not religious - just spiritual', sought guidance through collective meditation sessons. 'We asked God what to do - we kept getting vague messages about water.' This clicked with some of Gandhi's writings. The Indian philosopher insisted on cleaning out his own toilet at his ashram despite protests from his high-caste followers. He once wrote: 'If we did not cherish false notions in the name of religion, we would never tolerate such filth.'
The Australian 'white sahibs' not only ran into suspicion from the slum-dwellers who resented this intrustion into their most private habits, but also from Madras's officials. The volunteers were drawing attention to the city's failure to provide even the most basic sanitation for many of its 5 million inhabitants. The Australians had also become a local attraction; hundreds gathered to gape but few onlookers were willing to wield a brush.
Enter M B Nirmal, an irrepressible former bank manager, who protected the Australians from seething city officials. He also convinced the Australians to lower their sights - to Mambalam sewer. Meandering through the slums, the sewer had not been cleaned of its effluent for 40 years and was a source of unspeakable diseases. Mr Nirmal and Mr Hendry stood for a week in the stinking open channel. They even lowered a table into the muck and held a banquet. Soon afterwards, Mr Hendry got typhoid.
'At first everyone was happy to watch,' Mr Hendry said, 'but now the people of one street will take over where the others leave off.' Ever the realist, he added: 'About 50 kids a day are swarming in. Of course we feed them if they work. Maybe that's why they come.'