Inflows into Sydney's dams have plunged to levels far below the previous worst drought but water demand barely budged last year, slashing storage levels by almost a third.

Since the drought started to bite around the Sydney region about 33 months ago, 281 billion litres have flowed into Sydney's biggest dam at Warragamba. The tally is less than half the previous lowest level for a similar period during 1939-41 at 628 billion litres, WaterNSW figures show.

Inflows into Warragamba Dam and other reservoirs serving Sydney have plunged to well below those of the previous record low. Credit:AAP

The Upper Nepean catchment, which accounts for most of the rest of Sydney's storage, counted inflows of 109 billion litres compared with 178 billion litres during that comparable 1939-41 stint.

The year-end readings reinforce the urgency of curbing water use. Warragamba, which makes up about four-fifths of Sydney's storage, was 42.9 per cent full as of Saturday, down from 62.9 per cent a year ago, WaterNSW said.