Drove past Shawtec today and saw a heap of secondhand chipboard. It's about 10-12mm and about 1200x800. It looks like they were cover sheets on the board they use to make office furniture, so they are a bit dirty and some have rough edges, but could be used for shed lining or internal parts.

Shawtec Pty Ltd
143 Francisco St
Belmont
(near the intersection with Abernethy Rd)