If you are unsure where to start, engage a valuer – they have the resources to research sales (via RP Data etc), undertake searches at the LTO for registered leases, connections with agents/managers/owners etc. The quality of the report, the detail of the information provided are indispensible providing that they have been briefed correctly. This info may not come cheaply, I have been quoted $12K+ to get a market rent (I am particular about who I use) but stuck with a very tight timeframe (reflected in the val price).
As for free information – virtually non-existant, you will have to develop networks with leasing agents or do the hard yards yourself talking to tenants (discarding their bs) or undertaking searches ($) – not all leases are registered so it is not a simple task for the unannointed.