Whale watching from Heron Island?
 

I'd love to go out to watch the whales while I am on Heron Island in early August.

Is it possible, or will I need to go on a separate ship departing from Hervey Bay?

moxie - I don't think the management on Heron Island, cater for Whale Watching, unless your out on a dive boat, and happen to pass one in your trip. Places like Hervey Bay, have heaps of boats that do whale watching, both half day, and full day trips.
Whales have already been sighted in Hervey Bay, just a week ago. The reason Hervey Bay is popular with whale watchers, is that there is a good chance of seeing whales, as the whales stop in Hervey Bay (sometimes the pod can stay with weeks), with calves, to rest them, prior to returning to Antartica.
Most other coastal whale watching operations, only see whales as they pass by.

As of the last time I was on Heron Island (2001), there were no boat trips organized specifically for whale watching. But, in season, you can often see whales from shore, especially on the side (SW?) of the island facing the Wistari Channel. Here they are forced quite close to shore due to the narrowness of the waterway between Heron and the adjacent Wistari Reef.

As tropo says, you may happen to spot whales from the dive (or snorkel) boats. On one occassion, my wife and I were snorkeling from a boat in the Wistari Channel when the boat operator signaled us all to come back to the boat. Turns out we were all swimming within yards of a humpback whale. When we got on board, the whale dove right underneath the boat, and, in so doing, stuck his tail fin up in the air right in front of us. Awesome!

We also saw whales from a greater distance on the boat trip from Heron to Wilson Island.

In season, Moreton Bay near Brisbane is another good place to go. We did a spectacular whale watch trip from Tangalooma on Moreton Island several years ago (in July) and saw humpbacks breaching repeatedly.