Any time of the year is suitabel to travel to Lord Howe Island. Daytime temperatures rarely exceed 26 c in summer or fall below 16 c in winter.
Sea temperatures are simliar.
These are great months for walking and many people do swim. There aren't many street lights so a torch is essential. A raincoat, shoes for reefwalking, walking shoes, wincheater or sweater. Dress for dinning is smart casual. Pack carefully as there is a weight limit, above this may incure an extra surcharge.