There seem to be barely any about in our area this Summer, in the usual places I have found them in previous years.
We're having an extra hot Summer in New Zealand's north, so perhaps that is also a contributor. I have also noticed that much of their habitat has been cut back, by well-meaning humans (please leave the grass verges to grow long, people), which in turn forces the butterflies to move location - and of course cutting back their habitat kills their eggs and any caterpillars feeding on the plants.
I also note many Asian paper Wasps cruising the blue butterflies' habitat plant Birdsfoot Trefoil, leading me to believe that these wasps are also hunting blue butterfly eggs and caterpillars, just as they do with Monarchs (and also similar for Copper butterflies - plenty of Asian Paper Wasps cruising their habitat too).