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A Brightly coloured group of young people is gathered on the exposed south end of the island, armed with spades, a crowbar, staves and wheelbarrows. Much chattering and giggling! A sharp westerly whips up the white horses on the harbour, and I see my own hands go from purple to blue and back again.
No-one seems to mind the cold, though. Everything is blue, sparkling in the winter sun, wind-washed and sun-washed. Obviously, people are enjoying themselves.
Why the crowbar, though? Well, it's a rocky, exposed ridge that's been chosen for this tree-planting.
Each time the spades go in, they grind, crunch, clunk on to stones. Our mentor, seasoned Forest and Bird campaigner Ken Mason, explains why this bony ridge has been selected. Usually, you go for good soil where there's protection from the wind.
But this year it's been a particularly wet summer, so it's a unique opportunity, as winter sets in, to plant bushes on what is usually unpromising, bone-dry soil. And we find that the soil is indeed moist.
Lena, the German girl in the party, has her work cut out saving worms - which have flourished in the moist soil - from being chopped in half by her spade.
Because it's normally so dry, though, our mentors explain that we will have to take special precautions with this planting: dig good and deep, prise out the stones, pour in water crystals, collect friable soil to tamp around the roots, then pop in a fertiliser tablet as a final votive offering.
Many of the young folk know the routine already. All around me, the surface grass is being removed, holes dug, stones ferreted out, stakes hammered in.
A monocultural stretch of sheep pasture is being quietly transformed into a variegated, exciting new area. A number of the low-lying, divaricated bushes being planted carry berries.
Some of these plants have not been on the island for almost 200 years. So we're restoring ancient patterns of growth.
Just as at the other end of the island the two-storeyed married quarters, which date from quarantine days and which for decades deteriorated into a sad, tottering ruin, have been miraculously returned to their ancient glory.
Just about, anyway. The next step is to fix the flooring. The spade and crowbar-wielders are mainly school pupils or other young people, some from polytech or university.
A few are on St Martin Island (or Quarantine Island) for the first time and are absolutely stunned by the peace of the place, the beauty of the views up and down the harbour. It's only 10 minutes by boat from Back Beach at Port Chalmers, but it's a different world once you arrive.
The vast vault of the sky. The crying of the gulls. The thump of the surf from Aramoana. The exposure to wind and rain and sun. And the agenda set by the sea rising and falling, the eyelid of the tides opening and shutting, as the island song puts it.
Most of these young folk know one another well, turn up regularly at least once a month, are members of the community. They goof around, enjoy themselves, relate well to the older people, including the two 80-year-olds working this weekend.
On this freezing day, three of the teenagers even went in for a swim, their chattering teeth playing kettle-drum at our communal lunch around the big tables. But mainly they come to work, work, work.
And why? Because they care about the environment. Because they know we don't have much time any more to turn our world around. Most of them can't vote yet.
So they vote with their energy and enthusiasm and anger at a government and the army of indifference which doesn't begin to get it and clutches at the vain hope the problems of global warming will go away.
These young folk are into regeneration. They know that to go forward we have to get back to a simpler, holistic way of living. Across the country, gatherings are being held, and in many of our schools, highly clued-up groups are forming, future scenarios are being beaten out on the anvil of despair.
When the St Martin Island community was founded more than 50 years ago, it talked about the need to walk the talk, to meld together work and worship. Today, using different language, the same pertains. Mere talk achieves nothing.
We need to yoke thought and action together, create living symbols of hopefulness, live out parables of community, and in a cold, westerly wind point the way to a richer, simpler, sunnier future.
- Peter Matheson is president of the St Martin Island community.