The NSW Heritage Act 1977 defines 'relics' as:
‘Relic means any deposit, artefact, object or material evidence that:
(a) relates to the settlement of the area that comprises New South Wales, not being Aboriginal settlement, and
(b) is of State or local heritage significance’
There is no definition in the Act for ‘work’.
Essentially ‘works’ are structures. They are ‘public works’ that are not buildings e.g. bridges, dams, roads, etc. ‘Relics’ are chattels. So there is a difference of scale.
An archaeological site will generally contain a mixture of structural remains from demolished buildings and works, plus the ‘relics’ including artefacts and deposits.
For example, a well on a site would be a ‘work’ but the artefacts inside it would be ‘relics’ and be managed as such under the Act via an excavation permit or exception.
This is explained further in the NSW Heritage Branch guideline Assessing Significance for Historical Archaeological Sites and Relics.