Another fare bucket question, this time domestic.

I've been monitoring flight prices across a week in May and August into two regional airports.

This morning the August flights all went up by $15 and $26. Last month a similar thing happened to the May period I was monitoring and all those fares have remained at the higher level.

Is this a price bump by Air NZ or do they just "short supply" the lower fare buckets across the board causing this price increase? As it affects the lowest available fare across the entire month (in fact most of the year).