Until the middle of the last century, Sydney was dotted with market gardens, many of them run by Chinese immigrants.

Now these isolated productive green patches are being swallowed up by the inexorable march of urban sprawl as Sydney spreads like a water stain to the base of the Blue Mountains.

Chinese immigrants came to Australia to mine gold, but when mining became less profitable and the great slump of the 1890s hit, thousands of them turned to market gardening - not just in Sydney but all around the country.

In Darwin, in the Northern Territory, they grew the bulk of the tropical town's fresh produce, while in Queensland they cultivated banana trees, sugar cane and lychees.

By the early 1900s around a third of Chinese living in New South Wales, Victoria and Western Australia were market gardeners, assiduously planting and harvesting everything from European cabbage to bok choy. Operated on a largely co-operative basis, the allotment plots often brought together a dozen or so migrants from the same clan.

As on the goldfields, Chinese market gardeners were resented by their European counterparts, who in 1900 set up a Market Gardeners' Association to try to squeeze them out. White Australians accused the Chinese of fertilising their soil with human waste and of living in unhealthy conditions.

But urbanisation proved a greater enemy to the Chinese than protectionism and xenophobia, and today only a few market gardens survive.

One of the most central is at Kyeemagh, sandwiched between Sydney's international airport and the suburb of Rockdale. Healthy looking rows of emerald-green herbs and vegetables are bounded by terracotta-tiled bungalows and blocks of flats. Some families have been working this land for 30 years, or more, supplying choy sum and Chinese spinach to Asian restaurants and groceries.

In recognition of its historical importance, the Kyeemagh market garden was placed on the heritage list of New South Wales in 1999.

'It is important to preserve them because we only understand who we are by knowing where we've come from in the past,' heritage consultant Meredith Walker said yesterday.

'There's also an aesthetic pleasure in market gardens, almost like vineyards. They enhance the natural environment. There's pleasure in seeing the order they bring to nature. They are really attractive.'

While the survival of market gardens in inner-city suburbs is probably assured through heritage listing, those on the fringes of the city are under far greater threat to their futures.

Already home to about 4 million people, Sydney is bracing itself for a big population increase in the next few decades.

Further westward expansion is blocked by the Blue Mountains, so housing estates are being built on the northwest and southwest fringes of the city.

The estates are encroaching on farms, woodland and market gardens. In the northwest, the suburbs of Marsden Park and Kellyville will have to accommodate another 60,000 houses.

Another hard-hit area is Bringelly, on the city's southwestern edge, which until recently provided Sydney with 75 per cent of its fresh produce.

Today it is a small, scattered population, but plans are for intensive development - housing tens of thousands.

'The only things that might secure some of the market gardens is the fact that they are on the flood plains of the Nepean and Hawkesbury rivers, so building on them would be risky,' Mr Walker said.

But for many market gardeners, the advancing tide of suburban life means a sad end to a tradition stretching back to the earliest days of Chinese settlement in Australia.