Australians could be forced to pay up to $500 million to help clean up Chevron’s old oil site on Barrow Island in Western Australia.

Chevron stopped production at the site in May after nearly 60 years. The company is now working on shutting it down, which includes sealing around 900 oil wells and restoring the land. The total clean-up cost is expected to be about $2.3 billion.

Government documents show that taxpayers may have to cover part of the bill because of a 1985 deal that lets Chevron claim back some of the clean-up costs from the royalties it already paid.

So far, it’s estimated that Western Australia might refund Chevron about $129 million, and the federal government could return about $387 million. But the exact amount is still unknown.

Critics say it’s unfair for taxpayers today to pay for a deal made decades ago. They believe Chevron should be paying more, not getting help from public funds.

Chevron says it and its partners will cover most of the costs, and any refunds will follow legal rules.

While Chevron’s oil site is closing, its larger gas project on the island is still running.