Each answer is a place, landmark or attraction in Oceania, one per country. The answers traverse all countries by crossing maritime borders (or close enough). Each clue is missing a country name from another clue (not the answer to that clue, just the name of the country).
Notes:
- Some long names have been shortened. For example, if the full place name is "Xyz Royal National Park", the answer will be just "Xyz" (often with "royal national park" or similar as the definition).
- While many of the answers are pretty obscure, all of them are notable enough to have a separate article on the English Wikipedia.
- In addition to the 14 Oceanian sovereign states, I have included the semi-independent Cook Islands and Niue as well as four other countries holding territory in Oceania: Chile, France, the United Kingdom and the United States.
1. Heads cut in Iran for war, anarchy's starting; {2} briefly turned to crater (4 6)
2. Baby Tony misguidedly goes around {8}'s easternmost embayment (6 3)
3. Beach village tribe's initiation ritual unfinished, brought back near {11}'s capital (7)
4. Mother stands by {7} finishers in wild arena's large marine area (5 5)
5. Lay topless after bola is spinned around both sides of {9} landform (5 3)
6. Crater lake becomes extremely hot one after heart of {16}'s capital is surrounded by turbulent lava (3 4)
7. {12} loses heart, insanely follows mother to volcano (8)
8. Following introduction to forestry in {3}, one area placed next to nearly futile conservation zone (8)
9. Volcanic group is the eastern half of {13} behind... Stay back! No, team leader! Await! Cut it out! (6 7)
10. Climb atop western part of {4} mountain (5 4)
11. Finally, you fail replacing leaders in {17} rock formation (5)
12. {20} briefly left crater's bottom after Python obtained unrestricted entrance to volcano (5 5)
13. Two principal lawmakers associated with {6} to cut out duplicated lagoon (5 5)
14. Trail for famous Gorilla – a path behind {10} finalist (6 5)
15. Jewish leaders, {19} couple, young fanatic beginners and second lieutenant tremble, having extreme love for body of water (9 4)
16. West of central {15}, point an angry official's head to archaeological site (3 5)
17. Water for a western {5} drinking establishment next to e-commerce corporation (7 3)
18. Museum to unusually obtain emu associated with {14}'s island, initially (2 9)
19. One family in the heart of {18} returning to all-round nuclear test site (6 5)
20. Seaside, Gabe unexpectedly embraces girl in front of {1}'s leaders (5 5)