Incorporating Tasmania into a visit to Sydney or Melbourne is something many overseas travellers don't even contemplate - after all, it's overseas (well that's how many people see it, even though the flight from Sydney to Hobart is only 20 minutes longer than the flight to Melbourne). The perception is that it takes time out of one's travel schedule and money from the travel budget to fit more distant destinations like Tasmania in, but with a bit of careful planning and application of a bit of local knowledge, such is not the case. In fact there is a way to include a trip to Tasmania into a visit to Sydney that could well be cost you less money than if you stayed in Sydney! Not only that, you'll get a day or two sightseeing in Melbourne thrown in for free!
Just as you can reach Tasmania from Sydney without losing any daylight time travelling, and at a cost that can often be cheaper than staying in Sydney, visitors to Melbourne can do the same thing. The secret is to take the overnight ferry from Melbourne to Devonport. From there, you can hire car and drive to Hobart, or catch the coach direct from the ferry terminal.