...definition of a Trigger Notam, and if possible Jepp / ASA reference?
Sure.
Australian
AIP GEN 3.1 paragraph 4, AIRAC system
4.3
Documents and charts issued in accordance with the AIRAC cycle become effective at 1600 hours UTC on the day prior to the nominated date unless otherwise notified; eg, a document with an AIRAC date 26 Jun becomes effective 06251600UTC (ie, during Australian Eastern Standard Time, becomes effective at 0200 hours local on 26 Jun).
4.4
At each AIRAC effective date, a Head Office
“trigger” NOTAM will be issued which identifies the elements of the AIP effective on that date and their respective “with effect” dates, plus any additional SUPs that become effective on that date. This “trigger” NOTAM remains in force until the next AIRAC date.
The idea is... you get your AIC and AIP SUPP in the post if you subscribe, or via internet if you don't. The Trigger NOTAM tells you they are now triggered to be active.