The objective of this study was the estimation of genetic correlation (r(g)) for milk production traits across milking frequency (MF) in New Zealand dairy cattle milked once-(OAD) or twice-a-day (TAD). Analysis was made of lactation yields of milk, fat and protein, and somatic cell score of Holstein-Friesian (F), Jersey (J) and crossbred FxJ breed populations cows herd-tested between 2008 to 2012 while in their first and second lactations. The estimates of r(g) were, in general, greater than 0.80, providing no evidence of a genotype-by-milking frequency (GxMF) interaction. Genetic correlations tended to be greater in second compared to first lactation cows and greater in J compared to F and FxJ breed populations. The results also indicate a scaling effect, due to differences in the genetic variances across breed populations and MF, with the variances being lower in the OAD population. The results demonstrate that there is no need for a separate selection scheme for the OAD milking cow population, as long as there is similarity between breeding objectives and selection indices for both MF systems. The scaling effect indicates that the best animals in TAD system would also perform the best under OAD but genetic progress is expected to be lower in OAD compared to TAD systems because lower genetic variances in breed populations milked OAD.