The mid-tier gold producer spent A$23.2 million last month on 80 million Echo shares at an average price of 29c per share for a 16.4% stake.
Since then, Northern Star has been making regular on-market purchases, totalling around $4.76 million for a further 13.7 million shares.
The average purchase price equates to around 34.5c per share.
Echo shares closed at 38c on Friday, a 52-week high.
Northern Star now holds 19.2% of the gold developer.
Echo is close to completing a bankable feasibility study for its Yandal gold project, based on a recently released maiden reserve of 15.6 million tonnes at 1.7 grams per tonne gold for 856,000 ounces, which wil...