Yes, a unicycle is do-able for commutes. 8km is not a great distance anyway, even on a unicycle.
You have not mentioned if you already ride a unicycle so I will assume not.
Time and distance on one is required to make it look easy and practical. As is an appropriate machine.
A large wheel job is harder to ride, you will need a lot of distance under your belt to ride one well. For example a 30" wheel is far more difficult to ride than, say, a 24" and you are not going to get anything near the 20% time savings that the application of high school physics may suggest.
Commuting takes more time than a bike. Work on an eventual speed of around i km every 10 minutes, 8 minutes a sprint. However for the short distance you are talking it is the time at each end of the ride is more significant anyway. That end time will be much as it is for a bike. And a unicycle enables you to easily mix it with public transport or being picked up by our girlfriend in her Micra. Great also for taking overseas and on air transport.
Carrying gear? Just a backpack, with puncture kit, pump and water bottle on the seat post. Again it takes time to get used to a backpack but not a lot.
Winter? Same as for a bike, it's suitable clothes and appropriate layers and lights for the dark.
In most states you will not need a helmet (nor is one relevant in most circumstances), and you can ride on local roads and on footpaths. South Oz is a little different so be aware of your state's requirements. Reflectors, brakes and so forth are also optional.
When on roads be visible just as you are on a bike. Clothing, flashing lights, etc. Though from considerable experience it is clear that when riding a unicycle motorists will seldom not notice you in daylight even when they'd miss a bike. Plus you will not be subject to punishment passes and antisocial actions that bike riders are subject to. It is very safe to ride a unicycle if you are as good at it as most bike riders are at riding a bike.
Unicycles are the sort of thing that people buy, give up on quickly, and then resell at an easy price. BUT there are gotchas in both new or second hand unicycles. So if you come across one that you think is a goer you can refer it to me for an opinion. The things that make a unicycle good or bad or suitable or not are not necessarily related to branding or price. For your commute purposes from as low as $50 on the internet to a couple of hundred. Don't splash out on an expensive unit until you know the nature of the riding that you will do, the tyres and seats wear out just as fast as on a cheapy and generally they are the same to ride. The fit out and backpack/lights/clothing will be much more.
Mine (below) is a basic nothing special "trainer" fitted out for purpose that has done nearly 50,000kms and it is still the right one for the job.