I enjoyed the two parts of this TWHS inscription. Each country deserves I think at least 1 WHS and for Tonga the ancient capitals can be one for sure.

Ha'amonga ‘a Mau'i Historic Park: This park has two structures. A trilithon and a stone which is said to be the kings throne. They are over 10 centuries old from the time the capital was located there. I had a look for maybe 20 minutes as that's all that it takes. But it's interesting to see how on such a small island in the middle of the pacific this was constructed.

Ancient Royal Tombs of Lapaha in the village of Mu'a: These tombs are a kind of stepped pyramids. There are several and all those burried there are in line of the current king. In the current capital even, the more recent kings, are buried in a similar way.

After reading the UNESCO submission I do agree and that this for sure has the potential for a WHS. 

How to get there? Best option is from Auckland to Tonga. At the time of my visit I only could fly back (or to Australia) but now you can go to Samoa as well with a weekly flight and continue to other islands.

(photo copyright CTB Global - permission to use on worldheritagesite.org)