Sydney's dams are now lapping at about 77 per cent, they dropped 0.8 per cent in the past week, and are down from 94 per cent in June 2017, WaterNSW data shows.

This compares with Cape Town, which was predicted to reach "Day Zero" in late April, triggering authorities to cut general water supply and force residents to get water rations from trucks. Now, the city predicts that it will reach this crisis point on July 9.

The driest place in Sydney over the past nine months has been Horsley Park equestrian centre, where 238mm fell between June 2017 and February 20, a Sun-Herald analysis of 37 weather stations found. Bankstown airport, which recorded its lowest January rainfall for at least 20 years, ranked second for lack of rain with just 243mm.

"Part of the reason for the lower rainfall totals may be that 2017 saw the fewest number of east coast lows since 2003," Mr Martin said.

"East coast lows are major rain-producing systems for NSW, with 60 per cent of all major dam-filling events in the Sydney catchments attributed to [them].

"Sydney's outer south-west suburbs have been drier than other parts of Sydney both in the most recent nine-month period, and in the long-term climatology, for example, the 30 years from 1961 to 1990," Mr Martin said.

One of the reasons for this is the areas around Campbelltown and Camden are "rain-shadowed" by their geography.

"The ranges along the coast south of Sydney between Wollongong and Heathcote rise up to 300-400 metres, whereas the North Shore hills don't rise up more than about 200 metres, and are less continuous, so don't provide that same [rain] shadowing of the inland areas."

Sydney Water spokesman Peter Hadfield said if Sydney didn't receive any rain at all and water consumption stayed at current levels, which are very high, it would only take about 21 weeks for the dams to ebb to 60 per cent, but this was "extremely unlikely".

"Spikes in water use during hot weather are driven by external use, such as watering gardens," Mr Hadfield said.

During January daily water use was 24 per cent higher than the 10-year average for the month. Overall during 2017 the daily water demand was 14 per cent higher than the 10-year average.

Sydney Water hydraulics operation manager Robert Ius said January and February's exceptionally hot weather had pushed up water demand on top of an exceptionally dry period from July to September.

"Personal [water] use per day [315 litres] is the same as it was in the 1940s," Mr Ius said. "Similarly, total water use for all of the 4.8 million Sydney residents and businesses for the financial year 2016-17 was around 588 billion litres.

"This is about the same volume as in 2003-04, despite Sydney's population increasing by around 1 million people since then."

HOW TO SAVE MONEY ON WATER

Water your garden in the early morning or late evening to avoid evaporation.

Add a 7.5cm layer of organic mulch to keep your soil moist.

Sweep or rake driveways and pathways – don't use a hose.

Choose plants suitable for your area.

Group plants with similar needs so they all get the right amount of water.

Improve the condition and water holding capacity of your soil by digging in some good quality compost or composted manure.

Lift the blade setting on your mower to allow extra growth during the hotter months.

THE BIG DRY

Last year Sans Souci Public School set a new record low annual rainfall of 577mm, well below the previous record in 1936 of 608.3mm and almost half the average total of 1080.9mm.

2015 Summary statistics

​Wettest overall = 1586mm at Frenchs Forest (Frenchs Forest Rd)

Driest overall = 784mm at Milperra Bridge (Georges River)

2016 Summary statistics

​Wettest overall = 1545.6mm at Castle Cove (Rosebridge Ave)

Driest overall = 796.4mm at Penrith Lakes AWS

2017 Summary statistics Wettest overall = 1427.2mm at Berambing

Driest overall = 508mm at Milperra Bridge (Georges River)*

*One day of missing data