The Botanic Gardens and Government Domains Employees Index has just become available on microfiche at the NSW Archives. If you are heading there, this is an index worth checking.
These records are for the period 1863 to 1926 and shows employee's date of birth, date of employment, status, salary and adjustments to salaries, or a change in duties. Occasionally marital status, housing allowances, leave of absence, pensions and dates of death are given. At the front of each volume there is an index, so it should be quick to find any ancestor who worked there.
One thing to remember is that the archives staff are very knowledgeable, you can ask them where you may find one of your ancestors. On my next visit I'm going to get some help with my Grandfather who I know worked for the railways in NSW at Everleigh and also at Woy Woy. Those records are difficult to negotiate!