"It's very low and it's set to get lower because the metropolitan strategy includes large population expansion in the areas currently used for food production," said Professor Bill Bellotti, who holds the Vincent Fairfax Chair in Sustainable Agriculture and Rural Development at the University of Western Sydney.

"The councils do value agricultural land and put in place plans to protect that land, but this is not supported at all at state level … we seem to be locked into this urban sprawl, McMansion-type development," he said.

Professor Bellotti used figures from the Rome-based Food and Agriculture Organisation on Australians' food energy intake to estimate that 3 to 5 per cent of the produce grown on Sydney's fringe ends up in the average resident's diet.

But the NSW Government's metropolitan strategy barely acknowledged food production, allowing development to gobble up farms despite the need for sustainable agriculture to deal with climate change, he said.

Under the strategy, about half the current food production in the city's south-west and north-west, including vegetables grown in greenhouses around Liverpool and the Hawkesbury, would disappear, Professor Belotti said.

"The proportion of water devoted to food production is also declining in the competition for its urban uses. So the situation for food production is not pretty in the basin," he said.

Professor Bellotti will deliver his findings at a UWS conference on Sydney's conflict between food and land hunger on Thursday.

Very efficient supply chains set up to deliver food direct to supermarkets had broken the link between the urban consumer and rural production, leading city residents to undervalue agriculture's importance, Professor Bellotti said.

Metropolitan fringe farms kept urbanites in touch with the produce of the soil and were environmentally important because they used recycled water and the city's green waste which can be converted into mulch and compost, he said.