Erin Brockovich sat in an empty hall, awaiting hundreds of people who would soon stream in from a cold California night seeking answers and reassurance, and gave a sigh of familiarity.

“I’ve been doing this for 20-plus years and I’ve learned that it won’t get better – it’ll get worse, and they’ll need to know that Superman is not coming to rescue them.”

The environmental activist was referring to the leak at a gas storage well in Porter Ranch that has pumped planet-warming methane into the atmosphere and displaced thousands of people, prompting California’s Governor Jerry Brown to declare a state of emergency for the neighbourhood.

Brockovich made her name two decades ago investigating groundwater contamination in Hinkley, California, a lonely battle in a desert hamlet, but this latest fight is 120 miles south-west on the edge of the San Fernando valley, just outside Los Angeles, and in a national spotlight. “Here it’s large and very public and it’s hard to get to know everyone personally.”

She intends to try, saying the stakes demand it. “It’s a huge disaster. It’s like a volcano that keeps erupting and won’t stop. I call it the BP oil spill on land. I think we’ll see the fallout for many, many years.”

That was a chilling prediction for the estimated 500 people who filed into a Presbyterian church in West Hills on Wednesday night to hear Brockovich and others brief them about the leak, and for Southern California Gas Company, which faces mounting criticism as it struggles to plug a leak which flared in October and shows no sign of stopping. The crisis has raised an outcry over the company’s decision to not replace a safety valve which was removed in 1979.

The sight of Brockovich, 55, a striking figure in shiny black boots, leather coat and beanie, will not have reassured the company, a division of Sempra Energy of San Diego, considering the fate of the first power giant to cross her path.

Her investigation into Pacific Gas and Electric’s contamination of Hinkley’s drinking water with hexavalent chromium led to a $333m payout, the US’s largest settlement in a direct-action lawsuit, and the 2000 film Erin Brockovich, which won an Oscar for Julia Roberts in the title role.

Also not reassuring for the company: Brockovich lives in Agoura Hills, just 20 miles from Porter Ranch, and considers the crisis to be in her backyard. Worse, she said the leak has made her sick the five times she has visited Porter Ranch, most recently last week.

“The first time I went I started woozing around within 10 minutes. I felt my throat was having something snatched out of it. My chest started to get heavy. It felt like I had socks shoved up my nose. It’s happened every time I’ve gone up there.”

Erin Brockovich: ‘I think we’ll see the fallout for many, many years.’ Photograph: Noah Smith/The Guardian

The leak, buried deep in a giant gas field that stores energy for California, now accounts for about a quarter of the state’s daily methane emissions. Methane is a powerful greenhouse gas which fuels climate change. Campaign groups say the broken well has been pumping out the equivalent in carbon dioxide emissions of 7m cars a day.

Health officials say the gas does not cause long-term health problems but that chemicals put into the gas so humans can smell it can cause sickness.

After trying and failing to plug the leak gas company crews are now drilling two relief wells which in theory will meet where the well emerges from rock that forms the capstone, which seals the gas in place. Southern California Gas said the wells may not be ready until late February, or even later.

Several thousand of Porter Ranch’s 30,000-strong population have left in recent weeks, citing headaches, dizziness, nausea and nosebleeds, and concern that there may in fact be long-term health implications. A crash in property prices is another worry.

Brockovich represents Weitz & Luxenberg, a New York-based law firm. It has signed up more than 600 people for individual lawsuits and expects that to grow to more than 1,000, said Christopher Dalbey, another representative. Brockovich has given Weitz & Luxenberg a celebrity edge over two LA-based law firms which have signed up hundreds more.

“She’s Erin Brockovich. She’s fixed things in the past,” said Judy Resnick, 70, who moved with her family of six adults and four dogs to a hotel last month. The activist sets people straight about her Hollywood avatar, said Resnick. “The first thing she said to us was: ‘I’m not Julia Roberts.’”

Resnick, a retired educator, said the leafy, planned community had been a wonderful place to live. “It was heaven. Everything right there. A beautiful park. The YMCA for swimming. Starbucks.”

Her daughter Hope Tropper, 49, a school secretary, said some people were less sensitive to the smell. “But not me. I was so weak the week of Thanksgiving – that’s when I was really, really sick.”

Southern California Gas Company is paying for the family’s stay in a hotel suite four miles away and the family expects it to pay to clean and treat their home once the leak is fixed. But they worry about long-term health damage. “My concern is what will happen in 20, 30 years,” said Tropper.

Another resident, Kaylee Jeon, 36, said her five-year-old son suffered abdominal pain, nosebleeds and nausea before the family relocated to Agoura Hills last month. “I’ve been sick too – nausea, vomiting.”

The family had sunk its savings into the Porter Ranch home and now worried if it would ever be habitable or sellable, said Jeon. “We sacrificed so much. No one told us there was gas in the ground.”

Jeon, one of Porter Ranch’s many Korean-born residents, turned up at the town hall-style meeting in West Hills, safely beyond the stench at Porter Ranch, to hear what Weitz & Luxenberg’s team had to say. The nurse had not yet chosen a law firm, nor heard of Brockovich – the real or screen version – until recently. “I’ve just rented the DVD.”

Weitz & Luxenberg said it held its meeting in a church because it was the most practical venue but the altar lent Brockovich and her colleagues a quasi-preacher air as they briefed an audience which had trekked here on a cold, wet night.

Bob Bowcock, an environmental investigator, drew gasps when he said the official estimate of 50,000 kilograms of methane escaping every hour may be a gross underestimate. Weitz & Luxenberg thinks it may be closer to 146,000 kilograms.

Local residents, most from impacted areas, listen intently to Brockovich’s speech. Photograph: Noah Smith/The Guardian

Brockovich told the audience it was the victim of an “ongoing assault” and to brace for a long fight. “If you don’t have the truth you can’t protect your family so we will work diligently to get the truth for you. It’ll take time. Think of it as an onion. You have to peel it back..”

She invoked her earlier fight with another energy company. “In Hinkley I remember a representative telling me the green water and two-headed frog were normal.” Porter Ranch, she said, was enduring something profoundly abnormal. “This is the single biggest air disaster I’ve ever worked on.”

Asked about Brockovich’s accusations, representatives for Southern California Gas said the company was posting daily information and had set up a dedicated microsite with the results of air sampling, the status of the operations and details on how to file claims for temporary relocation and reimbursement.

“We know the community and our neighbors in Porter Ranch are frustrated, and we are highly motivated to end the impact on the community and the environment as quickly as possible.”

The statement promised to plug the leak – by spring. “With our relief well efforts, we’ve drilled to a depth of approximately 5,400 measured feet (we have to drill to a depth of over 8,000 feet), and remain on schedule to complete this process between late February and late March.”