About 1,000 households will be chosen to take part in the study, said to be first of its kind in the city. It will be officially launched next month.

In addition to a questionnaire to find out how much they know about water conservation, all household members will be asked to fill out a logbook for a week to record how they use water, including how long and when they bathe, and how often they use a washing machine.

The Water Supplies Department, which commissioned the study, hopes the consumption patterns gathered can allow more effective planning for conservation measures.

Water consumption rose steadily from 212.7 litres a person a day in 2003 to 221.7 litres in 2007. The figure has remained at about 220 litres in recent years - roughly 30 per cent higher than the world average.

The department's assistant director for development, Bobby Ng Mang-tung, said increasing water charges was not a priority.

'Increasing water charges may deter people from using too much water,' he said. 'But the effect can be short-term.

'Water is a basic necessity, and we hope there would be a general consensus in the community before we considered [raising charges].'

At present, tariffs for domestic users are calculated in four tiers. The higher the usage, the higher will be the charges per unit.

Roughly 46 per cent of households pay water charges of HK$25 or less a month, while about 21 per cent pay more than HK$75.

Water charges comprise just 0.3 per cent of the average household's total monthly spending, according to department figures for last year.

The water consumption study will be conducted by the research consultancy MVA Hong Kong and will last until December. A pilot poll, covering about 30 families, will start this month.

$0.00

Cost of the first 12 cubic metres of water a month.

- The next 31 cost HK$4.16 each, and the next 19 cost HK$6.45 each.