The indicated resource jumped by 136% to 331,000 ounces of gold at 4.84 grams per tonne, while the inferred resource was up 97% to 667,000oz at 4.59gpt gold, for a total 998,000oz.
OceanaGold CEO Mick Wilkes said the company had made prudent investments in extensively drilling out the mineralised structures beneath the Martha pit over the past two years.
"Since that time, we have reported dozens of significant drill intercepts and a growing resource which has now culminated into an indicated resource of 331,000oz of gold and inferred resource of 667,000oz," he said.
"The consenting (permitting) process was completed nearly a month ago and we continue to add significant resources as we seek to achieve our exploration targets.
"We are in the process of implementing the project development and mine plans. Once fully developed and ramped up, production from the Martha underground is expected to increase to recent historical levels while delivering significant socio-economic benefits to New Zealand."
The company also reported spectacular drilling results from Waihi.
Results included 10.7m at 36gpt gold and 221.8gpt silver; 4.7m at 47.2gpt gold and 67.9gpt silver; 16.5m at 10.1gpt gold and 17.1gpt silver; 1.3m at 70.7gpt gold and 71.7gpt silver; and 4.1m at 19.8gpt gold and 14.6gpt silver.
OceanaGold said the results supported its previously announced exploration target of 5-8 million tonnes grading 4-6gpt gold.
"The recent drill results at Waihi continue to demonstrate the significant exploration potential at the Martha underground," Wilkes said.
"We will continue to invest in our extensive drill program from the two underground drill drives and have allocated a budget of over US$11 million for 2019 to go with the $8 million planned spend for our newly discovered WKP prospect which we reported an initial resource of 234,000oz of gold in the indicated category and 401,000oz of gold in inferred resources."
Guidance for Waihi this year is 60,000-70,000oz at all-in sustaining costs of $875-925 an ounce.
OceanaGold shares dropped 1.4% to A$4.30 in Australia today.