The airline, which ends its financial year in June, said it would provide more details on the impact of the changes when it issues half-year results in late February.
Qantas has not made material capacity adjustments for the fourth quarter, it added.
The airline said it would run 70% of its pre-COVID-19 domestic capacity in the third quarter, down from a prior forecast of 102%, at a time when Australia is reporting record numbers of COVID-19 cases due to a runaway Omicron outbreak.
International capacity will fall to 20% of pre-COVID-19 levels from 30% due to tighter travel restrictions in countries including Japan, Indonesia and Thailand, Qantas added.
"We have the flexibility to add capacity back if demand improves earlier than expected, but 70% still represents a lot of domestic flying and it's a quantum improvement on the levels we faced only a few months ago," Qantas Chief Executive Alan Joyce said in a statement.
He said early bookings for the Easter holidays in April were promising for domestic and international flights.
(Reporting by Jamie Freed; Editing by Clarence Fernandez and Christopher Cushing)