Pension fund AustralianSuper has increased its stake in Origin Energy to 16.50%, the Australian power producer said in an exchange filing on Tuesday.

The stake increase by AustralianSuper, Origin Energy's top shareholder, threatens to scupper a $10.5 billion buyout of Australia's largest energy retailer.

AustralianSuper is opposed to the takeover offer by the Brookfield-led consortium and its partner EIG, saying the offer is substantially below its estimate of Origin's long-term value.

On Monday, the pension fund rejected an "eleventh hour" offer from the consortium to drop its opposition to their bid and join the takeover.

(Reporting by Sameer Manekar in Bengaluru; Editing by Subhranshu Sahu)