The 23-year-old played a crucial role in Tonga qualifying for the World Cup - to be held in Australia in October-November - but maintains playing for the Kangaroos is his ultimate goal.
City coach Tim Sheens said selectors had sought clarification on the rules regarding eligibility before naming Mateo, saying they didn't want to put pressure on the Eels utility to make an early call on his rep future.
He now has until next month to decide, when a berth in the NSW side would rule him out of Tongan contention.
But it seems Mateo's mind is already made up, even if he was unaware he remained eligible to play for Tonga today.
"I said I want to represent Australia first, I believe if I play this game I can't play for Tonga at the end of the year so I guess it's Australia or nothing this year," Mateo said .
"I hadn't really thought about (rejecting the City selection) to tell you the truth, I'm just really proud to make this team.
"I'd love to (play for Australia) ... I guess it's in my hands if I play well enough."
Mateo's unavailability would be a major blow to Tonga's World Cup campaign - and the credibility of the World Cup - coach Jim Dymock having realistic expectations of making it through to the semi-final stages.
"I haven't talked to Jimmy for a while now, I guess he might be upset a bit," Mateo said.
"I might get that call soon."