Auckland's water supply dams in the Waitākere Ranges have plunged to only around 30 per cent of their capacity, as dramatic pictures emerge showing one dam that has almost dried up since February.

Photographs taken from the same spot on February 7 and May 8 show the Upper Nihotupu dam, off the road to Piha, has shrunk in just three months from near-normal water levels to exposing huge empty slopes between the tree line and water.

Titirangi resident Scott Downie, who took both photos, has lived and walked in the area all his life and has never seen the water level so low. He is 33.

"In my living memory, I have never seen such a difference," he said.

Dael Armstrong-West, who was with Downie when he took the latest photo on Friday, said she was "gobsmacked".

"We couldn't believe it. We almost cried," she said.

Watercare's website shows that the water level in the dam in the week ending May 3 was just 30 per cent of capacity, the second-lowest of Auckland's 10 water supply dams.

The Upper Nihotupu dam on May 8. Photo / Scott Downie, Supplied

Watercare media liaison adviser Maxine Clayton said it was now down to 29 per cent.

The other four dams in the Waitākeres ranged between 29 and 36 per cent of capacity at May 3.

However, the average across all 10 Auckland dams was 46.3 per cent because five dams in the Hunua Ranges were at between 42 and 100 per cent of capacity.

Watercare says there was only 22mm of rain in the Waitākeres in the seven days to May 3 compared with a historical monthly average of 160mm, but the Hunua dams received 60.5mm in the week to May 3 against a historical monthly average of 149mm.

Overall water levels across all 10 dams is now 45 per cent of capacity compared with a historical norm for this time of year of 75 per cent.

The Upper Nihotupu Dam on February 7. Photo / Scott Downie, Supplied

This is the lowest average since the summer of 1993-94, when water levels fell below 40 per cent and severe water restrictions were imposed, leading to the decision to build a pipeline from the Waikato River to boost Auckland's supply.

"We haven't had any rain over the weekend - it was gorgeous weather but there's a downside to that," Clayton said.

Downie said the sight of the Upper Nihotupu dam brought home the reality of the crisis.

"I've heard in new news media about lowering water reservoirs, but seeing it has put a perspective on it for me personally and made me really watch what water I'm using, so I've cut showers down and I'm not wasting water," he said.

Watering gardens, washing cars and waterblasting will be prohibited from May 16 in the first of three planned stages of escalating restrictions if there is no significant rainfall.

Stage 1 will also prohibit the watering of sports fields, plants or paddocks unless an irrigation system is fitted with soil moisture or rain sensors, and require car washes to operate only if they use recyclable water.