The probable reserve stands at 130,000 tonnes at 5.2% nickel for 6800t of contained nickel.
The indicated and inferred resource for the project was updated to 168,000t at 9.5% nickel for 16,030t of nickel in August last year.
"This healthy Silver Swan Reserve upgrade is an important and significant benefit from the ongoing derisking and restart studies on and around Black Swan operations north of Kalgoorlie," interim CEO David Riekie said.
The company also reported a newly defined electromagnetic anomaly beneath the Black Swan open pit.
Riekie said the anomaly would be a "focal point for a priority drilling program".
"Collectively, these outcomes are important additions that are building on the certainties created from previous successful drilling, to allow reserve status and upgrade, as well as highlighting potential within previously untested zones," he said.
"The proximity and location of the anomaly is an exciting prospect."
Assays from exploration at Windarra also confirmed both nickel and gold mineralisation.
Reverse circulation drilling at the Crazy Diamond prospect returned 5m at 0.75% nickel from 12m, including 1m at 1.38% nickel, as well as 1m at 0.51 grams per tonne gold from 30m and 2m at 0.45gpt gold from 33m.
Last week, former Panoramic Resources managing director Peter Harold started at Poseidon as MD and CEO.
His arrival coincided with the stock hitting a 52-week low of A2.8c.
Poseidon shares last traded at 3.1c, valuing the company at nearly $82 million.