Flight's been changed (it's a secret)

The return leg of my journey between Osaka and Auckland was booked in July and then changed by Jet Star in September. Whether it be from overbooking or a dropped flight (I doubt it in Xmas time), I don't know. I also very nearly didn't know that they'd even changed it. I had a missed call which I assume was from them (unknown landline number), but no message was left and I never got another call from them. Sure I could have called them back, but who ever heard of a company that would field customer calls like that?

I also received an email from them on the same date as the assumed call. It said "Jetstar Flight Itinerary" and had a pdf with the attached flight with the changed dates. Essentially it looked exactly like the mail I'd received when I booked the first time, hence why I nearly assumed that they'd just accidentally sent the same thing twice (maybe they weren't sure if they had the first time). I ignored the double-mail. Only a bit later I had an afterthought and checked to be sure, that I saw the flight was moved to the next day (which is a big deal when you have to get leave at a Japanese company, which is hardly easy)

My complaint:
- Nothing was ever conveyed to me by phone or email that my flights had been changed. This means I have to rebook my connecting flight in NZ as I hardly want to sit in Auckland airport for a day.
- Nobody checked to see I got the message of a MAJOR change to my itinerary.
- Nowhere on any communication ("what communication") did anyone express the company's apologies for the inconvenience.

I've had bad service from their cabin crew before but chalked it up to a single person being bad at their job. But this goes much deeper than a bitter stewardess, this is costing me (and supposedly others) time and money, and somehow neglecting to even communicate this fact.

International
New Zealand-Auckland
Japan-Osaka (Kansai International)