The Chinese don't do anything for free.
They get their pound of flesh.
Not only will they get Tonga's UN votes, I'm sure substantial fishing rights were agreed too as well...
I guess the big risk in the Pacific is that if AU and NZ isolate Fiji too long, it will allow China to get a strong(er) foothold there too.
Why fight a future war when all China needs to do is spend huge amounts of money to achieve their long term strategic goals.
Look at Aviation, China has a growing list of Aviation interests bought for a song in the USA (TCM engines, Cirrus, Lake etc etc).
I guess India is slowly following China's lead with ownership of Airvan and an interest in Piaggio Aero...