Stewart Dawson

Hello to everyone,
I am new to silver collecting so I have a lot to learn and I’m hoping that someone might be able to help. While in England I purchased a set of coffee spoons with a sugar spoon included. It looks to be Art Deco or maybe Art Nouveau. It is in a nice flat box with blue satin on top and velvet lining on the bottom. The label inside says Stewart Dawson & Co. (NZ) Ltd. The top left reads Auckland/Wellington, the bottom right Dunedin/ Christchurch. The spoons are marked in order with what looks like an IP/maybe the image of a tree and an NS. The first and last mark are in an oval and the middle mark is round. I am hoping that someone could help me decipher these? What era were they made and are they silver or silver plate? I have read that Australia did not have any type of system for marking silver the same way England and the USA do, and I’m not even sure this is correct. Any information would be truly helpful. I can also try to post an image if that helps. There are six spoons on each side and the sugar spoon in the middle. Thanks to all.