Am I allowed to leave Houston airport during a long layover on an international connection?

For any landing in the US, you have to go through immigration, no matter what your further plans are - there is no concept like 'Transit Area'.
After that, you are inside the United States, and free to go anywhere you like.

In your case, immigration is actually in Toronto already (so be early at the airport!), and you are technically 'in the US' before you even board your plane. The rest is a 'domestic' flight, there is no immigration, passport control, or customs when you land.

Houston, Texas, has two major airports. If you're flying from YYZ to SYD by way of Houston, you're almost certainly flying through IAH, otherwise known as George Bush Intercontinental Airport.

As the other answers mention, once you're past immigration, you're free to go wherever you like; however, as a longtime Houston resident, I will give you a local's perspective on leaving IAH to go sightseeing.

IAH is approximately 35km north of downtown Houston, and is surrounded by industrial, residential, and semi-rural land. There's not much to see out there. What sightseeing there is tends to be farther south, away from the airport.

There is no train system that connects to the airport. The only public transportation out that way is the Metro bus system, whose route planner says the trip between IAH and downtown takes between 1 and 1.5 hours each way.

Houston has both Uber and Lyft, as well as traditional taxi service, but the drive is still a half hour minimum, up to an hour or more in rush hour traffic.

Driving is the primary means of transportation for most of the region's 6 million inhabitants, and many of them are on the roads at the same time. If you absolutely must see downtown Houston, be sure to leave plenty of time for transit to and from the airport.

If you're still determined to see something of the area on your layover but aren't sure you can make it to downtown and back, I can suggest the Mercer Arboretum, a large park devoted to cultivation and care of plants and trees native to the area. The arboretum suffered great damage during Hurricane Harvey in fall 2017, but reopened in March 2018. Mercer is only 15 minutes northwest of the airport, and you should be able to get there, visit, and get back to the airport in plenty of time.

Source: I've lived and driven in Houston for multiple decades.

If you clear customs I suspect you can leave the airport.

I live in Houston and not many sites near the airport. You could rent a car and go to Lake Houston but it is not much to see.

There is an hourly bus that goes into downtown. From there you can catch the train and see a lot of Houston. It goes through the medical center and down to the football stadium.

From the train you can stop at the museums, Herman Park, or zoo. Living in Toronto I don't think you would like Houston heat so I would suggest the museums. I like the HMFA.

Between IAH and downtown is Greenspoint. Don't get off the bus. Nothing to see and high crime.

As far as dining I don't know of much by IAH but a lot in downtown Houston.