Curiosity got the better of me and a quick check of the...

Curiosity got the better of me and a quick check of the interwebs revealed this ASIC guidance note (https://download.asic.gov.au/media/5770142/rg5-published-27-august-2020.pdf).

It's a legal geek-fest and I certainly didn't read all of it (heaven forbid!). However, some targeted keyword searches quickly revealled these nuggets:

What:
"Persons who, together with their associates, have relevant interests in voting shares representing 5% or more of the votes in a listed company [i.e. a substantial holding], body or listed registered managed investment scheme, must disclose details of their relevant interest. Disclosure must also be made when a person’s substantial holding changes by 1%, they cease to have a substantial holding or they make a takeover bid." (p.6. Emphasis added.)

Nothing new in that. Highlighted here to introduce the term "relevant interest", which ties-in with the next snippet.

When (Trigger):
"A person acquires a relevant interest at the time of entering into an agreement or understanding, or being granted enforceable rights or options to acquire securities, regardless of the time for performance or exercise, or whether any conditions are attached." (p.10. Emphasis added.)

So, the trigger is the date of the purchase contract (i.e. on-market trade date, or date an off-market agreement is made).

When (Notice Timing):
"Where the listed entity is not the subject of a takeover bid, the substantial holding notice must be given within two business days of:
(a) the person becoming, or ceasing to be, a substantial holder in the listed entity; or
(b) a movement of at least 1% in the person’s substantial holding in the listed entity."
(p.73. Emphasis added.)

Example:
"Remember that a person may acquire a relevant interest in securities (and therefore become a substantial holder) before an agreement is performed [eg: settled].

"For example, a person who makes an ordinary purchase of securities on-market will acquire a relevant interest in those securities when the trade takes place, not when settlement occurs (usually three days later)." [Z edit: I think this was written in the T+3 settlement era, before moving to T+2]
(p.73. Emphasis added.)


So, there you go, @tarasbulba. By my reading, any new 5%ers would need to lodge by their F603 by today (i.e. two business days following breaching the 5% relevant interest threshold, anchored from the purchase contract date, not settlement). That's not to say that every holder is fully compliant all of the time, but in my experience the pros are generally pretty good with their disclosure compliance obligations.