AirHint found the following cheapest return flights from Seattle (SEA) to Sydney (SYD) in December 2025 (see full year).
Return flights
| 02 Dec - 01 Jan | United | ||
| 11 Dec - 23 Dec | Frontier Airlines | ||
| 14 Dec - 21 Dec | Air Canada | ||
| 22 Dec - 08 Jan | Hawaiian Airlines | ||
| 21 Dec - 06 Jan | Cathay Pacific | ||
| 14 Dec - 09 Jan | Delta |
Also check the deals for one-way flights from Seattle to Sydney in December.
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