I have travelled extensively throughout the South Pacific, and believe I have a few handy tips regarding your accommodation and choice of resort/island. Firstly, Fiji is a great first choice for travellers. You usually don't need any vaccinations (but always check with your travel doctor, just in case), and the currency of the Fijian dollar is very similar to the Australian dollar. My first choice in Fiji is Castaway Island, a great island for couples, honeymooners or families. This island is in the Mamanucas, so you will probably need one night either side, due to the time the boat takes, and the time your plane lands in Nadi. Usually you will take a coach to Denarau Marina, and then you will get a launch from there, to the island. As the launches only operate in daylight hours, this means you usually need to stay one night at the airport hotels. The Tokoriki Hotel or the Tanoa Hotel are excellent choices for this, they offer great facilities and accommodation, considering they are airport hotels.
Another great choice if you don't want to stay out on an island is the Shangri-La Fijian resort. This is just huge, and has many choices of restaurants, pools and activities. This is my favourite, I have been there 3 times. Another popular choice is the Outrigger Reef Fiji, close by to the Fijian. These Coral Coast resorts are very large, and cater to everyone.
In Vanuatu, if you are a honeymooner, I would definitely recommend Iririki Resort. It is on it's own island, and the accommodation there is just gorgeous. If you are travelling with your family, I recommend Crown Resort (formerly La Lagon Park Royal). This has a nice calm lagoon area great for kids. I recommend pre-booking a breakfast or meal package before you arrive in Vanuatu, due to the French influence, the food prices can get quite high. Better if you can pre-pay as much of it in Aussie dollars before you go.
Rarotonga is another great option - this island is very small, you can travel round the whole island in about an hour! They have great little motorbikes that you can hire, or jeeps. The Sokala Villa's are my personal favourite here, they are A-frame bungalows, each with their own private pool (except 2, which have direct beach access), and they would be great for a honeymoon. If you are travelling with a family, then The Rarotongan is a great choice.
Enjoy, please remember to check each resort on the website before making your decision to go. The majority of them have interactive websites so you can view the rooms and facilities before you even step into the airport! Your travel agent also is a valuable information guide, the majority of them will have travelled there. Bon voyage!