Under a binding agreement, Yancoal will acquire a 10% stake from joint venture partner Sojitz Corporation for A$300 million.
The consideration is payable in four instalments. The first $50 million will be paid this week, followed by another $50 million by the end of June.
The third instalment of $100 million is due at the end of the year, with the remaining $100 million due in March 2021.
Yancoal will fund the acquisition from cash holdings.
"Yancoal views the proposed acquisition as an attractive opportunity to increase its participating interest in the Moolarben JV from 85% to 95%," the company said.
"The immediate benefit will be the increase in attributable revenue, which Yancoal will consolidate into its financial results from this low cost, tier one asset; and with potential for the Moolarben JV to increase its production profile in the future, subject to ongoing work with external stakeholders."
A consortium of Korean companies holds the remaining 5% of the JV.
Moolarben produced 20.5 million tonnes of coal last year.
The operation recorded a 2019 net profit before tax and extraordinary items of $1.53 billion.
Yancoal shares closed 1.5% lower at $2.07 on Friday, valuing the company at $2.73 billion. The stock is down from $2.80-2.90 at the start of the year.
