A statistical study is carried out to investigate the probable stress-strain properties of Grade 275 and Grade 380 reinforcing steel manufactured in New Zealand for reinforced concrete construction. The investigation is based on extensive measured data obtained from Pacific Steel Ltd and from University of Canterbury research projects. The results obtained from this statistical study are used to calculate the 5% lower-tail normal probability value of the yield strength, which is considered to be more appropriate for use in the strength design of reinforced concrete members than the specified minimum yield strength. The results obtained from this statistical study are also used to re-evaluate the values of the currently recommended beam flexural overstrength factors used in seismic design in New Zealand, namely 1.25 if Grade 275 steel is used as flexural reinforcement and 1.40 if Grade 380 steel is used as flexural reinforcement. For this purpose, moment-curvature analyses incorporating an appropriate stress-strain model for the steel reinforcement and the Monte Carlo simulation technique are conducted to study the likely increase in the flexural strength above the ideal moment capacity of reinforced concrete members at high curvatures. Improved values for the flexural overstrength factors are recommended.