Billionaire Andrew Forrest's stake in Poseidon Nickel has been sliced in half by the dilutive impact of the embattled junior company's strategies to conserve cash.

Mr Forrest, who chaired the nickel junior between 2007 and 2013, held a 32 per cent stake in the company in 2012, most of it gained by underwriting a $20 million rights issue that year.