THE amount of water used in growing food is not the only way to judge the "green-ness" of what we eat.

Less water is not necessarily better, CSIRO research leader in sustainable agriculture Brad Ridoutt told a recent Food Farming and Health forum in Warragul.

Dr Ridoutt said rating foods by the volume of water to grow and process them was too simplistic.

Water use should be measured across the full lifecycle of a food product.

Dr Ridoutt said it had been claimed that it took 75 litres of water to make a litre of beer, but 200 to produce a litre of milk.

"But so what if it took 200 litres to make that milk?" he said.

"What is the sustainability of that water being used? What is the nature of the water being used to make that milk?

"Is it taking water from the environment or using natural rainfall over agricultural lands?"

The virtual water content of a branded 575g tomato-based pasta sauce, for example, was 202 litres - seemingly less thirsty than a branded 250g pack of chocolate-coated peanuts at 1153 litres.

However, when researchers looked at water used in growing raw materials and at local water stress, they found a major component of the confectionery was cocoa grown in rain-rich Africa, while the tomatoes required water-intensive irrigated land.

Dr Ridoutt said the need to consider existing water demands on individual catchments was important in calculating future water supply.

"In many of the world's existing food bowls, water is being unsustainably used," he said.

Dr Ridoutt said the CSIRO was looking at water use across the full life cycle of different foodstuffs.

The forum was organised by Agribusiness Gippsland, a not-for-profit group working to support regional agribusiness.