How do I apply for a PNG visa to visit family in Madang please.?

You can find all the requirements on the official website of PNG embassy in PRC. You should provide:
1. invitation letter and authorization letter from PNG Immigration Bureau
2. copy of the business license of the company
3. health certificate
4. copy of the inviter's valid visa
5. life guarantee letter in PNG
6. non-criminal record paper, notarized marriage certificate (child's birth certificate should be notarized if you have a kid)
7. copies of your passport and ID card
8. visa fee: CNY 150

I think you'd better contact the embassy if you have futher problems.
Tel:8610）65324709，65324312