The document, among those leaked by former NSA contractor Edward Snowden and reported exclusively by the Canadian Broadcasting Corp., does not list exact targets.
U.S. President Barack Obama and many other heads of state and government were in Canada during the meeting, while protesters from both countries and elsewhere were also active.
The NSA briefing document says the espionage included "providing support to policymakers," the CBC reported Wednesday. Previous leaked papers suggest the Communications Security Establishment Canada worked with the NSA in eavesdropping during the London G20 meeting in 2009.
Snowden documents have also said the CSEC hacked into communications of Brazil's mining agency.
Canadian law bars the CSEC from conducting surveillance in the country without a warrant, regardless of whether targets are Canadian citizens or foreigners.
"If CSEC tasked NSA to conduct spying activities on Canadians within Canada that CSEC itself was not authorized to take, then I am comfortable saying that would be an unlawful undertaking by CSEC," Craig Forcese, a national security expert at the University of Ottawa, told the CBC.
The new document lists agenda items at the 2010 meeting, held while much of the world was still mired in an economic downturn. That suggests a major goal was to gain information that would help Obama and Canadian Prime Minister Stephen Harper in negotiations with foreign leaders.