Conditions: ‘The NGV Collection contains approximately 75,000 works of art and approximately 90% of its records are available for viewing here. More than 30,000 images of works in the Public Domain are available to download at high resolution for free for publications and non-commercial use by clicking on the download icon. We are constantly improving and adding to the information on this website through research and scholarship, with updates being made to records on a regular basis and images added when they become available.’
At present (June 2019) there appears to be no way to filter for free content in a search. As a guide, objects made by unknown artists, or over 100 years old tend to be out of copyright.
Source: search landing page