SYDNEY—A consortium led by Brookfield Asset Management made last-ditch changes to its near US$11 billion offer for Origin Energy but failed to convince Origin’s biggest shareholder to back the deal.

The revised proposal, submitted Wednesday evening, forced the Australian energy company to postpone Thursday’s Sydney shareholder meeting just two hours before it was due to start. Shareholders, led by superannuation fund AustralianSuper, were unlikely to sufficiently support the prior offer, said Origin, as it postponed the shareholder meeting to Dec. 4.