Complex sentence: Leichhardt's second expedition, now with the assistance of a government grant and substantial private subscriptions, started in December 1846 and was supposed to take him from the Darling Downs to the west coast of Australia and ultimately to the Swan River and Perth.
Split into simpler sentences: Leichhardt's second expedition, undertaken with a government grant and substantial private subscriptions, started in December 1846. It was supposed to take him from the Darling Downs to the west coast of Australia and ultimately to the Swan River and Perth.