Officers are on site conducting further checks of the paddock where the horses became sick. Among scenarios being examined by authorities is that a toxic weed sparked the deaths.
A spokesman said staff had also been speaking with neighbouring landholders as part of the agency's investigation.
The necropsies on the horses began on Friday. They are expected to be finished this week.
Microscopic examination of organ tissues from the dead animals is expected to reveal whether they died from an infection or a toxin.
Authorities were alerted to the deaths last Thursday.
Tests have ruled out the Hendra virus.