Most visitors to Australia need an eVisitor visa (for EU/UK passport holders) or an ETA (Electronic Travel Authority) for others, both applied for online before you travel. New Zealand requires an NZeTA for visa-waiver nationalities, also done online, plus an International Visitor Conservation and Tourism Levy. The Pacific islands vary widely — Fiji and Tonga grant visas on arrival for many nationalities, while French Polynesia follows French visa rules. Check requirements for each country individually; don't assume one Pacific island's rules apply to the next.
Australia uses the Australian dollar (AUD) and New Zealand the New Zealand dollar (NZD). Both countries are heavily card-oriented — you can tap to pay almost everywhere, including market stalls and small-town cafes. The Pacific islands are another story. While Fiji and larger towns accept cards, smaller islands and rural areas run on cash. French Polynesia and New Caledonia use the CFP franc. Bring enough local currency for a few days when heading to remote islands, and don't count on finding an ATM.
English is the primary language in Australia and New Zealand, and you'll have no trouble communicating across both countries. In the Pacific, things get more interesting. French is the language of daily life in French Polynesia and New Caledonia. Fiji is English-speaking, and most Pacific island nations use English alongside local languages. Learning a few words of the local language — even just greetings — goes a long way, particularly in smaller communities where tourism isn't the main industry.
Australia and New Zealand are very safe countries by any measure. The Pacific islands are generally safe too, with low crime rates in most areas. The real hazards in this region come from nature: Australia's wildlife reputation is earned (box jellyfish, saltwater crocodiles, snakes, and spiders are all present), but locals live with them just fine. Follow posted warnings, swim between the flags, and you'll be okay. In the Pacific, strong ocean currents and remote locations mean you should respect the water and not overestimate your swimming ability.
Mobile coverage and internet are excellent throughout Australia and New Zealand, including decent 4G/5G in most populated areas. In the Pacific islands, connectivity drops off sharply. Major towns in Fiji and Tahiti have reasonable internet, but on remote islands you may have limited or no signal. If you need to be reliably connected, this region will teach you to plan ahead — or to enjoy the break.