The State Library of NSW’s collection of Seidler drawings and diaries – and a lot of it is online.
The library for the Aboriginal Affairs department contains a number of books an journals about aboriginal housing in Australia. It is in Spring Street and is open to the public.
The SLQ’s entrance page to its Architecture collection – little appears to be online.
This architecture resource guide is a good way to start accessing the collection of the State Library of Victoria. Also try the database for photos of many old Victorian buildings.