[GRASSLIST:2508] Which projection?

[Gordon Keith]

I have a gridded data set I want to read into grass, but I'm not sure 
which projection to use. The header for the data contains the following 
information:

Datum: WGS84
Half axis: 6378137.0000000
Flattening: 1/298.25722356300
Coordinate system: Lat/Long
Latitude min.: -42.78126745
Longitude min.: 148.31130940
Latitude max.: -42.58820665
Longitude max.: 148.49252740
Latitude cell size: 30.00 meter
Longitude cell size: 30.00 meter

I can get the same data output using a different coordinate system with 
the header:

Datum: WGS84
Half axis: 6378137.0000000
Flattening: 1/298.25722356300
Coordinate system: merc
Y min.:  -5248244.68
X min.:   4966111.48
Y max.:  -5219451.16
X max.:   4985106.57
Latitude cell size: 30.00 meter
Longitude cell size: 30.00 meter

I'm not very familar with projections and don't understand the 
significance of the half axis and flattening terms.

The data is gridded, but the grid coordinates don't follow lines of 
latitude and longitude, so I haven't had good results reading the data 
into grass using a latitude longitude projection.

Could someone please suggest the projection and information I will need 
to create it in grass?

I will need to work with this data in latitude/longitude coordinates. 
Does this mean I will have to reproject (r.proj) the data into a 
latitude longitude projection, if I get it into grass successfully? 

Thanks in advance
Gordon