It could take years to reconstruct the damaged dam above the Derbyshire town of Whaley Bridge, residents have been told, as some of the 1,500 evacuated were promised that they would now be allowed to return to their homes.

At a community meeting, Derbyshire police said that residents of one part of the town could begin to go back to the village after water levels in Toddbrook reservoir dropped rapidly. But some still face waiting until after a Wednesday inspection to be certain that their homes are absolutely safe.

A spokesman for the Canal and River Trust at the meeting in nearby Chapel-en-le-Firth said that it was hard to know how long the reconstruction of the dam would take.

“We are very much in the emergency phase now and we are currently repairing and carrying out construction work,” he said. “It is a long-term construction project, but we will not have started from scratch. It could take 18 months, two years, three years, who knows?”

Rachel Swann, deputy chief constable of Derbyshire police, told the meeting that residents of the Horwich End area could return home.

Chinook helicopter carries sandbags to Whaley Bridge dam to carry out major emergency repairs. Photograph: News Images/Rex/Shutterstock

“I have got a meeting at 12pm tomorrow where I am expecting we will have good news,” she said. “We have obviously been pumping the water out and it has gone down at a fast speed. It is now beyond 9.5 metres. We will keep draining the water until it is safe to stop.”

Earlier emergency crews who had pumped 1.2m tonnes of water from the damaged reservoir were told they had reached their target.

The Canal and River Trust had said it needed to drop the water level by eight metres and confirmed the level was now down to 8.4 metres.

The trust said: “The water has been pumped out at a controlled rate and good progress is being made.”

The reservoir level currently stands at 25% of its holding capacity.

Nonetheless, Derbyshire fire and rescue service urged caution after residents were given false hope of being able to return to their homes.

In response to a tweet saying “the dam is considered safe”, the fire service said: “Nowhere have we said that the dam is safe now. Work is ongoing, and road closures and evacuations are still in place to preserve life.

“We will open roads and let people return home as soon as we can, but we have no way of knowing when this will be.”

Fire crews have been using dozens of high-volume pumps to remove the reservoir’s water since part of its spillway collapsed on Thursday afternoon following days of heavy rainfall.

As residents were evacuated an RAF Chinook helicopter was drafted in and began to drop hundreds of bags of sand and aggregate to shore up the slipped pieces of concrete.

Since then seven tonnes of water a minute have been pumped out of the structure, built in 1831, drastically lowering the threat of flooding across the valley down towards the towns and villages of the High Peak.

The water taken out of the reservoir has been directed into a sluice channel before being dropped into the River Goyt. The Environment Agency confirmed it was monitoring water levels in the river.

Meanwhile, work on the damaged dam was hampered by subsidence overnight. Tonnes of gravel and stone that had been loaded into the damaged section of the dam since Friday had “settled” resulting in the Chinook returning and dropping a further 50 tonnes of aggregate to repair the area.

Previously police and local authorities criticised around 31 residents who decided to stay in their homes within the evacuation zone, saying they were “taking their lives into their own hands” and jeopardising the safety of the emergency services.

Malcolm Venton and Lorraine Ellis told the Guardian they could not leave their two border collies, Meg and Amy. The couple were asked to leave on Saturday when the evacuation zone was extended, but decided to hunker down, despite the threat of a huge wave rushing out of the reservoir.

• This article was amended on 7 August 2019. A quote from the Canal and River Trust was misattributed to the Environment Agency in an earlier version. This has been corrected.