OAKLAND — Dozens of Oakland hills residents were erroneously charged for past fire inspections, prompting confusion and outrage and sending the city scrambling to correct the error.

Due to a fire department snafu, an estimated 1,000-plus invoices for reinspections dating back to 2012 — at $303 apiece — began arriving in mailboxes of hills residents the past few weeks. But the residents, many who took to neighborhood sharing site NextDoor to vent, said the inspections either never took place or the city never sent them prior documentation. The mass mailing was prompted by previous city audits that found an inept vegetation inspection and abatement collection system by the Fire Prevention Bureau.

Fire officials acknowledged this week they found errors in data entry and were attempting to inform some residents that their fees due May 17 will be voided. On Thursday, officials planned to extend the deadline to July 20 and conduct an audit of all invoices to figure out which ones should be voided, Vice Mayor Annie Campbell Washington said.

After years of failing to bill for inspections, the city stepped up its effort this year to collect an estimated $400,000 in back fees from noncompliant property owners, money that could help the fire department expand its fire inspection unit in the wake of the deadly Ghost Ship fire in December. Mayor Libby Schaaf’s proposed budget for the next two years calls for increasing the number of inspectors from eight to 20.

But earlier this week, Oakland Fire Marshal Miguel Trujillo said about 20 percent of the bills were sent in error. Trujillo, speaking at a city meeting, said “we did find some errors in our data entry” in the department’s maligned OneStep software. That program has been cited in numerous fire department complaints for creating confusion over inspection referrals to failures to track inspections.

Trujillo did not reply to an email asking for more comment. But the mistake could strip the fire department of its billing authority sooner than later. Schaaf and her fire task force had earlier recommended relieving the department of that responsibility.

“From what I can see so far, it is further evidence that we need to move the billing and collections function out of the fire department and into the revenue division,” Schaaf said.

The city administrator’s office late Thursday acknowledged that the billings “caused a great deal of concern and confusion,” for many residents and asked the community to support the fire department’s efforts to improve hills vegetation inspections.

One resident who posted on NextDoor said a city form indicated an inspection took place around midnight and others said the city never previously notified them of the inspections until the invoices arrived.

Barbara Levy, who lives in Redwood Heights, is one of several residents who said they received two invoices, for a total of $606. One of Levy’s inspections was said to have occurred at 6:53 a.m. and she doubted it ever took place.

The city eventually voided her two bills after one inspector looked and saw the database’s notes indicated they should not be billed for a reinspection, she said. Fire officials told her they did not take the notes section into consideration before mailing out the invoices.

Evelyn Sinclair owns two properties near Shepherd Canyon Road and said she has no records that align with the two reinspections in 2016 for which she was billed. So far her emails to the city haven’t been returned.

“What’s really disheartening to me is if I failed an inspection, this is actually important for safety reasons. You would think they would inform me. Wouldn’t that be the point?” she said.

A November 2013 city audit found problems with the fire department’s collection process for vegetation inspections, abatement and reinspections. The issues had not been fixed by the time by City Auditor Brenda Roberts conducted a follow-up audit in December 2015. In that report, Roberts recommended ensuring the fire department “invoices noncompliant property owners for all abatement costs, re-inspection fees and administrative costs annually.”

She also recommended improving the department’s billing system and for firefighters to team with the city’s Revenue Division to create a collections process to recover abatement costs. Between January 2011 and August 2015, the fire department did not submit a single vegetation management invoice to the collections unit, according to the audit. In fiscal year 2014-15, of the $150,000 in invoices mailed by the fire department, only $33,000 had been collected, the audit found. It was worse in fiscal year 2012-13, when $105,000 of the $127,000 invoiced had gone uncollected.