HI Angel,
Generally, we are conservative, but that is in the outer or rural provinces, and more so to the locals. In urban Centers, and the main provinces like Port Moresby, you can dress how you feel and want, as long as you are comfortable and it is decent. The apparel you mentioned is totally OK, there are many locals and expatriates that dress that way in urban areas.
Just for further clarification, where exactly is your husband being posted?
Hope this helps.
Cheers,