of interest I thought these 2 images show synoptically what we need for strong northerlies.
first image shows a high just off the nsw coast. so winds are strong to the SW where the cold front is.
as the high moves over towards NZ the cold front pushes NE and creates the killer northerlies we've been having. the stronger the high is over new Zealand the better the northerlies.
then as the cold front passes the coast we get the SE winds the next day. sometimes, the cold front turns into an ECL as it hits the coral sea. which is the image I posted above.