Ann: Postponement of General Meeting , page-22

I think Humble is right. The laws regarding when and how a board can postpone a meeting seems to be defined in the company constitution. Boral for example can change the date/time of a meeting to any date time. There is no need for 28 days notice:

http://www.boral.com.au/images/common/pdfs/Corp_Gov/BLD-Constitution.pdf

Seems to be similar things fore the ASX group too:

http://www.asxgroup.com.au/media/PDFs/asxl_amended_constitution.pdf