looks like you have it downpat already
From Zone football rules & regulations -
Fixtures will be played on the following basis –
Week 1, 4 v 1 Saturday; 3 v 2 Sunday
Week 2, 1 v 4 Sunday; 2 v 3 Saturday - applies to all Grades.

from my understanding the 3 assocaitons get together and allocate a ground from each area to host the Zone Grand Finals. And to provide a good venue they choose NPL standard grounds.
I havn't seen the allocations this year but looks like they may be out already going off some of the posts already up.