Planted some plants in flower box, there seems to be lots of dying leaves, though lots of new ones are sprouting and lots of buds for flower are appearing, is it normal for a lot of leaves to die through the season

Could be just transplant shock. Where are you? New Guinea's don't do well for me in Texas heat of summer.

First of all, Welcome to NGA! I was also going to ask where you are, and how much sun they are getting. They need morning sun and partial shade. They fry in heat and full sun. Here is some info I found:
http://www.gardenguides.com/78...

Hello thanks for the replies we live in Canada, southern Alberta, yeah I chose the impatiens because they would be mostly in the shade, so they just have a few hours of sunshine in the morning, I was thinking of transplant shock too, there is a lot of new growth from the stems, I don't know much about plants,i was just surprised to see so much foliage turning brown

I'm thinking it's transplant shock. It sounds like they should do well for you in your location. They are gorgeous plants!