The Cone Penetrometer Test (CPT) is the most commonly used in-situ test to evaluate liquefaction potential using empirical correlations developed over the last 50 years using case histories involving very young soil deposits. Recent studies have shown that factors like aging and bonding can increase soil liquefaction resistance through the contribution of soil microstructure. Paired cone penetrometer data (CPT) testing combined with shear wave velocity (Vs) testing (e.g. sCPT) can be used to derive a strength gain factor which evaluates the contribution of soil microstructure to liquefaction resistance. Additionally, correlations between the soil behaviour type index, Ic, calculated using CPT, and laboratory-derived plasticity index values are shown to have an effect on a soil’s susceptibility to liquefaction for young (non-aged) sands. This paper compares two methods to derive the strength gain factor for aged soil through measured to estimated shear wave velocities from seismic CPT data, with consideration of incorporating the soil behaviour type index, Ic. Paired testing results and laboratory data are reviewed to further refine the soil behaviour type index (Ic) cut-off values for aged soils in the Bay of Plenty, New Zealand.