Maroons media officer Ben Ikin said Hannay had come down with a viral illness over night and was "very crook" and on medication. Hannay had not yet been ruled out but Tonga was considered likely to take over at left centre in the Telstra Dome clash with NSW, even though he wouldn't arrive in Melbourne in time for the final training session this afternoon.
Tonga played two Origin matches for the Maroons in 2004 before an injury-plagued 2005 session. He recently returned from a hamstring injury and scored a fine try in the Bulldogs' 32-10 win over Wests Tigers at the weekend.
Meanwhile, each team will have their one and only chance to familiarise themselves with the Telstra Dome during closed training sessions this afternoon.
Neither side has been able to train at the venue for the series decider in Melbourne so far. Dome officials have been reluctant to have too much traffic on the ground following two AFL matches played there over the weekend.
The Maroons are also unlikely to have the roof closed for their final run with sunny conditions in Melbourne today. Telstra Dome staff will not close the roof if the sun is shining due to concerns over the state of the surface. NSW will hold its session after the Maroons at 4:30pm.