From what I understand with discussions with a few geo's - 5...

From what I understand with discussions with a few geo's - 5 mgal is relatively significant and indicative of a dense body - most probably an iron body.

IOCG deposits are iron oxide bodies with the iron being haematite which usually gives it a strong gravity signature. So the 5 mgal response is a good sign.

However I cannot find magnetic info on the gravity anomalies at Murninie.

You will often hear of IOCG systems having a co-incident magnetic anomaly. Whilst this is true - the magnetic signature is usually weaker as the iron in IOCG systems are haematite which does not of itself have a magnetic signature. If it did have a strong magnetic signature may indicate something else - i.e. magnetite.