Some options to consider...
To show the summed value for multiple selected suburbs, I would create a second worksheet that either had a list of suburbs with a grand total sum, or just a single value with sum of all selected suburbs. Either of these options would be driven by a filtering action where selections on the map would drive a filter to narrow which suburbs show up on the worksheet with the summed value. Filter Actions
If you're working with Tableau 10.5 or later, you could then embed that viz into a tooltip on the map...or you could just create a dashboard with the two worksheets side-by-side.
To show the combined region as one, take a look at the options for creating custom territories (Build your own custom territories in Tableau 10 | Tableau Software ). Another fun outlet for doing this might be to see what is possible with set actions in the 2018.3 Beta (out now, here are some of the new features (Coming Soon: New features in Tableau ) and check here if you want to download and try it out... https://prerelease.tableau.com/key/2018_3_beta )