Hi @Wombat,
Sorry for the delay in responding but i wanted to check my facts first.
The short story is that it's at the company's discretion.
You will find any criteria that the company may use to reject or scale back applications listed under 'additional information' in the letter you receive from the company about the SPP. Therefore it is not necessarily true to say 'first in best dressed' will get your full allocation, although in some cases it may help.
I have attached two examples, the ASX Listing Rules and also the ASIC rules related to SPP's.
Interestingly, in Example One (sorry i can't highlight the text as the PDF document is locked, just scroll down to additional information) the company gives several criteria they 'could' use when making a decision to scale back applications. Three are listed including: size of holding; whether you bought or sold shares after the record date; and the date on which your application was received.
Remember also that a company could use their 'allowance' under rules 7.1 and 7.1a to accept over-subscriptions without requiring shareholder approval.
Should a SPP be scaled back, money is usually 'returned without interest as soon as practical'.