In July, the company warned of a cost blowout of as much as US$1.9 billion on the $5.3 billion capital costs after challenging ground conditions required major changes to mine design and a delay of 16-30 months.

McIntosh told Rio's Capital Markets Day in London overnight the company had made "better-than-expected progress" since then.

The 1.3km-deep, 10m-wide Shaft #2 is complete and in the final stages of commissioning, which McIntosh said would be a game-changer.

Yesterday, the company received the permit for the service hoist, allowing 300 people to be moved via Shaft #1 via 60 previously.

September was a record month for productivity with 1385 equivalent metres of lateral development completed versus 1213m in August.

"Our teams now understand how to develop through the ground conditions they're facing," McIntosh said.

McIntosh was unable to elaborate on mine design work, which is now underway to allow delivery of a new mine plan in the first half of next year.

Rio remains on track to deliver a definitive estimate on the final costs and schedule in the second half of next year.

Rio owns a 33.5% indirect interest in OT through its 50.8% shareholding in Toronto-listed Turquoise Hill Resources (TRQ), which holds 66% of the project in joint venture with the Mongolian government.