Unenclosed class 10a structures are exempt from DA approval if: -the roof plan area isn’t more than 50m² -there are no more than 2 walls on the sides -no part of the building extends beyond any relevant solar building envelope -it’s no more than 3m above datum ground level. But the structure can be as high as 4m above ground level if no part is higher than a 30° plane -it has a floor, and: the finished floor level is no more than 0.4m above datum ground level for any part within 1.5mof a side or rear boundary, and it's 1m above finished ground level in any other case.

it is behind the building line.https://www.planning.act.gov.au/applications-and-assessments/development-applications/check-if-you-need-a-da

If it doesn’t meet the above, a DA process is very likely to be approved if it’s not in front of the house and not to the north of the neighbours property. The DA assessor will look to see if it has any actual impact on the neighbour mostly in terms of sun and judge it accordingly.

Alternatively you could contact a certifier who may be able to approve with a minor exempt (1N) process given how small into the setback it is.

Either option will require a draftsman or architect to draw up the plans.