If you can work from home, then you must do so. However, if you are a sole practitioner and you are the sole occupant of the office premises (ie not in a shared office) and it’s not ‘practicable’ to work from home, then yes, you may attend your office, provided there is no-one else in the office with you. See para from Vic Govt Table (Victorian Government Stage 4 Restrictions ‘Guide for Business – Stage 4 Restrictions):
‘Under Stage 4 ‘Stay at Home’ Restrictions the default is that workplaces in metropolitan Melbourne are closed unless the workplace is part of a permitted industry as set out below. All Victorians are required to work from home, except where this is not practicable. Sole operators can continue to operate, if they do not have contact with the public, or with people other than those persons living in their primary household.
The Guide for Business – Stage 4 Restrictions (Workplaces) provide examples of workplaces that are closed for on-site work, open for on-site work with a COVID Safe Plan, and where there are restricted operations or industry specific obligations.
This information is subject to the Directions of Victoria’s Chief Health Officer.