The problem with zeroing in on locations like a lake in a place like New Zealand is the umpteen number of such locations available for filming. All we get to know from movie related sites is that the film crew shot in New Zealand. So unless we visit these lakes personally, it becomes that much difficult to pin point the exact lake. But we got lucky in the sense that one of our friends who is in New Zealand recently visited the lake and was able to confirm that these songs below have been shot there at the lake. This being such a popular lake, we believe that the number of song sequences shot at this location is only going to increase.

About the location (from Wiki):
Lake Tekapo is the second-largest of three roughly parallel lakes running north–south along the northern edge of the Mackenzie Basin in the South Island of New Zealand (the others are Lake Pukaki and Lake Ohau). It covers an area of 83 square kilometres, and is at an altitude of 2,300 ft above sea level. The lake is fed at its northern end by the braided Godley River

The lake is a popular tourist destination, and several resort hotels are located at the township of Lake Tekapo at the lake’s southern end. The Lake Tekapo Regional Park, administered by Environment Canterbury, is located on the southern shore of the lake. An astronomical observatory is located at Mount John, which is to the north of the town, and south of the small Lake Alexandrina.

Movies shot at this location:
Thaen Thaen from Kuruvi
Cheliya Cheliya from Kushi (Telugu)
Oru Punnagai ThaanE from Theeradha Vilaiyattu Pillai
Aur Mohabbat Hai from Main Prem Ki Diwani Hoon
Nenjam Orumurai from Vaseegara
Rama Chakkanodamma from Rajakumarudu
Thee Illai song from EngEyum Kaadhal
MellinamE MellinamE song from Shahjahan
Kavvinchake O Prema song from Raja