The QGIS user manual shows how to work with projections, see http://qgis.org/en/docs/user_manual/working_with_projections/working_with_projections.html. In this instance you need to follow the Custom Coordinate Reference System section.
Of course for you to do this you need to know what the proj4 parameters are for the CRS you wish to create.
Looking at the NSW Map projections page (specifically the section on ISG - Integrated Survey Grid) and the linked page to Australian Geodetic Datum 1966 (AGD66), can I think take you most of the way...
For example AGD66 is based on the Australian National Spheroid
Semi-major axis (a): 6 378 160.0 metres
Semi-minor axis (b): 6 356 774.719 metres
Flattening (f): 1/298.25
In proj4 syntax these are:
+a=6378160.0 +b=6356774.719 +f=298.25
Looking at at the ISG - Integrated Survey Grid information we have:
Projection: Transverse Mercator
Zone Width: 2 degrees
Longitude of Origin: Central Meridian of each zone
Latitude of Origin: Equator (zero degrees)
False Easting: 300 000
False Northing: 5 000 000
Central Scale Factor: 0.99994
Units: Metre
Ellipsoid: ANS
In proj4 syntax these are:
+proj=tmerc +lat_0=0 +k_0=0.999940 +x_0=300000.000000 +y_0=5000000.000000 +ellps=ANS
+units=m +lon_0=[Central Meridian of each zone]
So now you just need to figure out how many zones there are and their central meridians, for that we can take a look at the Manual of the New South Wales Integrated Survey grid Table 1 (below image).

Putting that all together, for example ISG zone 54_2, gives us I think:
+proj=tmerc +lat_0=0 +lon_0=141 +k_0=0.999940 +x_0=300000.000000 +y_0=5000000.000000 +ellps=ANS +a=6378160.0 +b=6356774.719 +f=298.25 +units=m +no_defs no_defs <>
You could call this CRS any name you like, but elsewhere I have seen it called EPSG:102072 or AGD_1966_ISG_54_2.
I should stress that I am not a projection expert, so you should probably double check I haven't produced a clanger here (for example you should look at the proj4 syntax PROJ.4 - General Parameters, but certainly this is where I would start.