Foundation materials and conditions are extremely variable. Designers must investigate all bridge sites by careful boring and testing. In big bridges it is considered desirable to test by drilling every proposed pier position.

Earthquake risk affects the design. Monolithic construction is probably most effective against earthquakes, but it is not practicable nor economic to make all bridges in this way. Simply supported precast and cast-in-place concrete, or simply supported steel spans are often the most economical. It is now general practice to link together by some means all spans of a multi-span bridge to ensure synchronous movement of all spans in an earthquake and to prevent the ends of individual spans from coming off the tops of piers and abutments.

Prestressed concrete was virtually unknown in New Zealand before 1952. In the 1960s it has become one of the foremost bridge-building materials. The longest prestressed-concrete motorway bridge is the recently constructed Victoria Park Viaduct connecting Auckland Harbour Bridge approaches with Auckland urban motorways – a four-lane bridge, 2,520 ft long. Prestressing techniques have been mainly used in precasting components (largely beams) in the factory. The Ministry of Works has produced many standard plans to make factory manufacture easier. But designers are making intensive efforts to apply prestressing to continuous-span, cast-in-place superstructures; for example, in the Wanganui Motorway bridge.

New Zealand must now build one new bridge a day to keep pace with the demand.