What's the planning process for adding a toilet/shower to a garage?

You are intending to connect to the ACTEW drainage system therefore a DA & BA are necessary.

Your builder must organise inspections with your Certifier, the plumber and electrician ditto.

You do not say if your garage is free-standing or attached to the residence.

If attached to the residence I advise to ensure you get a certificate of compliance and guarantee from the tiler/person who does the waterproofing of the shower base, if it leaks later on and damages the residence you could have major repairs to contend with.

If the garage is free-standing there would be no concern about the residence but a leaking shower base could still cause damage if the garage is timber-framed.

Googles yr mate
http://www.planning.act.gov.au/topics/design_build/da_assessment/exempt_work/process/roofed_enclosed_or_open_on_1_side

You need to talk to a certifier, it may or may not need a DA depending on a number of factors. We went though the same process and the certifier confirmed in our case it was DA exempt.

In either case it will need plumbing certification and a new plumbing plan for the block submitted to planning. For us this was handled entirely by the plumber who did the work.

Note that this does not in any way make the garage now be classified as a habitable or secondary residence…. that’s a whole other process. From a planning perspective you will end up with a garage, and an additional external bathroom for the primary residence. (Which in our case was fine)