It is issuing 109mln new shares at a price of 0.55p each.
The company told investors that it is now focussing on maximising the delivery of short and medium-term benefits from its projects.
Moreover, amid the strong oil price, it intends to accelerate efforts to increase production at the Welch project.
Cash proceeds will support well workovers at Welch, driving more additional wells to come online.
It will also cover feasibility work for possible horizontal wells, and allow long lead items to be acquired ahead of a horizontal drilling programme.
Separately, the cash will allow Mosman to increase its stake in the Arkoma project, to 33.3% from 27%.
“Mosman has decided at this time an equity issue is preferred by the company rather than a debt facility,” the company said in a statement.
“Going forward the company expects to consider a range of funding alternatives when evaluating the horizontal wells at Welch, or longer-term development of the Arkoma project.”