Parachuting figures have been spotted dropping out of the sky near the landmark since soon after November 2008, when the stack was earmarked for demolition.

Port Kembla Copper resealed the welded entry to the abandoned stack base yesterday after discovering that it had been cut open.

Company general manager Ian Wilson said he began receiving reports of base jumpers near the site about three years ago, soon after someone cleverly replaced a link in the security chain with their personal padlock.

He said security staff once found a 400-metre rope at the site, thought to have been used to haul parachuting or related gear to the top.

"It's been reported by residents when they've been taking dogs for a walk in the morning; people have landed and there's only one place they could be coming from," Mr Wilson said.

"We've always ensured that the security of the base is as tight as possible so the [base jumpers] can't get through.

"We put thicker, stronger chain on, then they've pulled the pins out of the door hinges."

Metal bars were later installed at the entry, and cut through.

More recently the door was replaced with a steel framework and welded closed.

Yesterday, security personnel discovered a newly cut hole, big enough for a small person to squeeze through.

Berkeley resident Alice Scott said she had witnessed two base jumpers "fluttering down" near the site under cover of darkness in August last year.

"It was eight o'clock at night and I caught them in my headlights," she said.

The copper smelter was closed on July 28, 2003, after a $600 million spend on new technology did not deliver the expected environmental and operational gains. It was later declared a risk site.

The stack is patrolled at random by security staff and inspected for entry breaches once a week, according to Port Kembla Copper.