The 58 per cent increase takes the estimated size of the resource, in the Mid-West region of Western Australia, to 153 million tonnes.

Of this, 69.9 million tonnes or 45.6 per cent of the total resource is direct shipping ore, the most prized iron ore as it can be mined and shipped to customers without further processing.

The remaining 54.5 per cent of the resource, or 83.4 million tonnes, is beneficiation feed, which requires conventional crushing, grinding and screening.

Murchison is attempting a $748 million takeover of fellow miner Midwest, which has previously taken shots at Murchison's Jack Hills resource, saying more than half of it was less-desirable beneficiation ore.

Murchison chairman Paul Kopejtka said the resource update was "barely three months after the last reported update", which had confirmed the potential and quality of the project.

Murchison said in a statement yesterday that the direct-shipping ore would produce a premium-quality feedstock that was sought keenly by steel mills.

Sources close to Midwest said Murchison had delayed by about a week releasing the updated resource estimate, but sources close to Murchison said the company was taking the time to ensure its accuracy.

"Further resource potential exists at all zones and further resource updates will be announced as resource modelling and drilling continues," Murchison said in the statement.

"This interim mineral resource update ... provides Murchison with increasing confidence that the Jack Hills project will deliver a substantial mineral resource that will underpin its major expansion plans."