Tanzania's main opposition presidential candidate has vowed to review mining and gas contracts, and also scrap "unnecessary" tax exemptions for mining companies, if elected president of the east African nation come October.

Investors in Tanzania, Africa's fourth biggest gold miner with plans to develop huge new gas discoveries, have complained of shifting goal posts in contracts with the state. "My government will also eliminate unnecessary tax exemptions in the mining sector ... and use part of its stake in gas reserves as collateral for loans to finance construction of natural gas infrastructure," Lowassa told miningweekly.

Tanzania estimates that it has more than 55 tcf of recoverable natural gas reserves off its south coast.

BG, Statoil, Exxon Mobil, Ophir and Petrobras are however, players in Tanzanian energy. Lowassa, who boasts some popular support, quit as premier in 2008 over corruption allegations that he denies.

 "My government will levy lower taxes and fees to joint ventures between local and foreign investors, while foreign investors who are not in joint venture with locals will face higher taxes and fees," Lowassa said.

The government said in May it would review mining pacts to secure a bigger share of revenues. Former Prime Minister Edward Lowassa, 62, disregarded as the ruling party's candidate last month, switched to become the opposition coalition's contender.