All travellers will require a valid passport to enter the United States and your passport should still be valid for 6 months after you return from your trip. If you are travelling on more than one passport, please use the same passport for the entire trip.
The following information also applies to visitors entering the USA under the Visa Waiver Program (including Australian, New Zealand, and British citizens):
As of April 1, 2016, all travellers must have an e-passport to use the VWP. An e-Passport is an enhanced secure passport with an embedded electronic chip. E-Passports are issued by the proper passport issuing authority and must meet international standards for securing and storing information corresponding to the passport and bearer.
More information can be found on CBP.gov
All travellers wishing to enter the US under the VWP will need to hold a passport possessing specified security features. Visa Waiver Program requirements are:
The passport must have a machine-readable zone on the biographic page.
The passport must be an electronic passport with a digital chip containing biometric information about the passport owner.
It is your responsibility to ensure you have the correct passport and that there are sufficient blank pages for any visas required and for entry/departure stamps.
Many travellers to the United States (including Australians, New Zealanders and British) entering the country require a prearranged, approved Electronic System for Travel Authorisation (ESTA). This is obtained electronically for passport holders from countries eligible for the Visa Waiver Program. Travellers must apply at least 72 hours before departure and it can be obtained at the following official website https://esta.cbp.dhs.gov for a fee of US$21 (correct at time of writing, subject to change), payable by credit or debit card. It is very important to make payments through the official website.
Most people who have travelled to Iran, Iraq, Sudan, Cuba or Syria or are dual nationals of these countries, do not qualify for entry under the Visa Waiver Program and existing ESTAs could be revoked. You will instead need to apply for a visa from the nearest US Embassy or Consulate. Certain exemptions apply on a case-by-case basis. Bottom line: If in doubt, please contact the nearest US Embassy or Consulate.
All other passport holders should check with the relevant consul.