The reduction in the cut-off followed a review of processing and mining costs and as a result of the increase in the gold price from the A$1650/oz used when the last estimate was made - with the price above $2500/oz in more recent times.
Twin Hills comprises the 309 and Twin Sister deposits that combined contain 12.8 million tonnes grading 1.8 grams per tonne gold.
Mining is envisaged by both bulk surface and underground methods, with the latest resource estimate on focused on 309.
Resource upgrade work is said to be continuing.
GBM recently acquired Twin Hills to boost its resources in the Drummond Basin region for its "processing halo" strategy that it anticipates will involve an overall 2-3 million ounce resource that has the potential to transform it "into a genuine mid-tier Australian gold company".
It currently has about 1.6Moz to its name.
The company is led by Peter Rohner, described as a "world leading figure in metallurgy" and includes (on its board), well known North American geologist and investor Brent Cook.
GBM had $6.6 million cash at the start of the current quarter.
Shares in GBM were up 9% to 12.5c in afternoon trade, capitalising the company at $64 million.
