Maggie T. Sutrov showered, drank treated tap water and watered her garden before she learned that she should not be using the water in her home on Maui after wildfires devastated the island. Concerned about others making the same mistake, she quickly created a flier on water contamination from guidance she had found on the county’s website and worked with a pop-up community center to get the word out.

“Every day, people were showing up there going ‘What, I can’t drink the water? I didn’t know that,’ “Ms. Sutrov said. Three weeks after the fire, Ms. Sutrov and others are anxious to know when the island’s water will be safe.

“When is this over?” Ms. Sutrov wondered.

So far, tests have found no concerning levels of contaminants in the drinking water. But extensive testing is still needed, with access to most of Lahaina slowed by hazardous conditions and the search for human remains.

Some areas under the unsafe water advisory could be cleared to use their tap water in a couple of weeks, said John Stufflebean, director of the Maui County Department of Water Supply.

But experts and history suggest it could take months or years before the worst of the damaged areas have safe water fully restored.

“We have a way to go before we can say that it’s safe,” Mr. Stufflebean said.

The county first told people in Upper Kula and Lahaina not to use their water on August 11 shortly after the fire damaged water pipes as it sped across the land. So far, one water quality test on the northwest edge of Lahaina showed low levels of benzene, a chemical known to cause cancer, but it was within federal safety limits.

That is likely a clue to what more testing will find, said Andrew Whelton, a Purdue University professor who studied drinking water contamination following California’s 2018 Camp Fire and Colorado’s 2021 Marshall Fire.

“As you move closer to the middle of the water system where structures were destroyed, you’ll likely see higher levels of contamination,” he said.

Where homes and other structures burned, so did their interior pipes, along with shallow-buried exterior ones that connected them to the public water line, and the water meters, Mr. Stufflebean said. The utility’s networks of reservoirs, pumps, wells and treatment plants on Maui were not affected, and it is unlikely that main lines—thick pipes buried more deeply—burned, he said.

“What other places have found in fires is that the main lines tend not to get damaged because they are buried,” he said.

That was the case in Paradise, California, the city almost completely destroyed in the Camp Fire. Main lines buried several feet underground were OK, though water utility assistant district manager Mickey Rich said small sections were damaged when lost pressure sucked in smoke and contaminants. Seventeen miles of the town’s 172 miles of main lines were contaminated and await replacement, and the city is still replacing service lines five years after the blaze.

Kurt Kowar, director of public works and utilities in Louisville, Colorado, said it took just a week to get parts of the water system there back online after the Marshall Fire. But in more severe burn areas, it took months to assess the damage, including where contamination had occurred, and flush it from the system. The city distributed bottled water and set up refill stations to hold residents over.

“We’re almost approaching our two-year mark. And as we’ve cleaned up the properties and people are starting to rebuild, we are still doing protocols of testing service lines to verify there’s no contamination,” he said.

Mr. Kowar just returned from a trip to Maui alongside others from Louisville and Paradise where they met with the Department of Water Supply to share knowledge about what to test for and how to decontaminate the system.

“It was very emotional to see all that again,” he said of the damage. “It was kind of healing to be able to give our knowledge back and help them move faster or give them an idea of what’s coming in the future.”

Mr. Stufflebean’s department will soon expand the number of chemicals it’s testing for, he told a room packed with residents in Kula, a mountainside community about 24 miles from Lahaina, last week. The residents were told not to drink or bathe in their water until further testing is completed.

When Mr. Stufflebean mentioned that a map for the water advisory was on the department’s website, some frustration could be heard. “Some people do not have internet,” one person responded.

The county-run utility is stretched thin trying to restore the water system and does not have a lot of time for outreach, said Chris Shuler, a hydrologist at the University of Hawaii at Manoa, who lives on Maui. To try to fill the information gap, he is part of a team from the university’s Water Resources Research Center that has set up a community tap water monitoring program.

They offer free testing for 88 compounds to residents within the unsafe water advisory area.

“People want to know what’s in their water, but at the same time they just want to have information and help them navigate through this difficult time,” he said.

As with the county, the research center’s first round of results did not find concerning levels of contamination, but they have only just begun. They have more than 200 requests for sampling. Any significant results would be reported to the county.

Ms. Sutrov was lucky. Her family’s home, where she was born and raised, survived the fire while some of her neighbors’ homes did not. Now, as the island community still reels in the traumatic aftermath, her patience for slow information is waning.

“Everyone in whatever their role is is doing the best they can, but there are also distinct gaps that have been uncovered in this,” Ms. Sutrov said.

Mr. Stufflebean acknowledged as much at the community water meeting she attended.

“We’re enhancing our communication, which has not been terrific, I admit,” Mr. Stufflebean said.

This report contains information from The Associated Press.