The Toronto-listed, Brisbane-based company wants to issue up to 686.6 million shares at 15c a share in Canada, a 50% discount to its last price of 30c.
Nautilus' two largest shareholders, MB Holdings Company and Metalloinvest Holding, which hold a combined 48% of the company, have indicated their intentions to participate in the offer.
Proceeds from the offer will be used to advance the construction and development of the company's seafloor production system for its 85%-owned Solwara 1 copper project off the coast off Papua New Guinea.
Outstanding contracts for equipment associated with the system at the end of January was around $US52.5 million.
The company had about $52.2 million at January 31.
On February 1, Nautilus took delivery of three seafloor production tools from Soil Machine Dynamic's UK facility, which are on their way to Oman for wet testing.
The tools will be used to cut and extract high-grade copper and gold from the floor of the Bismarck Sea.
Nautilus plans to integrate the tools into its production support vessel next year ahead of the start of production in the first quarter of 2018.