UPDATE 1-Newcrest extends exclusivity period for Newmont's $20 bln buyout offer

(Updates with Newcrest comment on extension in paragraphs 3 and 6 and background on deal in paragraphs 4 and 5)

May 12 (Reuters) - Newcrest Mining Ltd said on Friday it has extended the exclusivity period by a week for Newmont Corp to complete its due diligence on its A$29.4 billion ($19.94 billion) final takeover offer for the Australian gold miner.

The exclusivity period for Newmont's best-and-final offer of A$32.87 per share, which was set to expire on Thursday, has now been extended until May 18.

"Newmont has substantially completed due diligence and Newcrest has agreed to extend Newmont's exclusivity ... on the same terms as previously announced to provide further time for Newmont to complete its due diligence and submit a binding offer," Newcrest said.

Newmont, already the world's biggest gold producer, did not immediately respond to a Reuters request for comment.

If the deal goes through it will be the third-largest transaction ever involving an Australian company and the third largest globally this year. It will lift Newmont's gold output to nearly double that of its nearest rival Barrick Gold Corp .

In April, Newmont revised its offer to 0.400 shares per Newcrest share after the Australian gold miner rejected two prior offers with exchange ratios of 0.380 shares and 0.363 shares, saying they undervalued the company.

"There is no certainty that the revised proposal will result in a binding offer for consideration by shareholders," Newcrest said on Friday. ($1 = 1.4743 Australian dollars) (Reporting by Sameer Manekar in Bengaluru)