But he said it is still "too early to make general conclusions".
Niger voters headed to the polls on Tuesday for the vote, which was opposed by opposition politicians and had sparked street protests in Niamey, the capital.
Hamidou said turnout was reported at around 45 per cent.
Tandja, who has ruled the uranium-rich Saharan state since 1999, wants three more years to oversee large foreign-backed investments including a Chinese oil deal and French uranium mine.
His second and, under the current constitution, final term expires at the end of 2009, a deadline which the country's highest court has said cannot be altered.
But Tandja has argued that the population wants him to stay.
In the lead up to the referendum, Tandja cracked down on his critics and the press in what his opponents called a "slow-motion coup".
"He took the oath of office swearing on the Quran to protect our nation's democratic institutions," Mahamadou Issoufou, an opposition leader, said.
"But instead, he is destroying them."
Issoufou had called on Niger's estimated six million registered voters to boycott the vote which he called "illegal".
In June, Tandja dissolved the constitutional court that had ruled against him and assumed the power to rule by decree, batting aside international criticism of he move saying he was answerable only to the people of Niger.
The European Union, a major donor to Niger, condemned the move.
"The dissolution of the constitutional court and the exercise of government powers without parliamentary control are grave violations of core democratic values and the principles of the rule of law," Sweden, which holds the revolving presidency of the EU, said.
The European bloc has already suspended the payment of one tranche of aid and warned that the president's pursuit of another term could have "serious consequences" for its co-operation with Niger.