Just spent the last 5 weeks in PG (Sabang to be exact), we were diving just about every day, even when the typhoon came in, one group went out in the morning, it was only a tropical storm with wind and some rain. The average afternoon wind in San Francisco in the summer is most typical of the wind I saw.
Anyway, so it's the rainy season, I have heard that the last 20 years in Asia, unless you are in a monsoon on the coast of Thailand or India, it never rains all day, sure you get a few days here and there, but the weather in PG was basically hot and humid the whole time I was there, with the water temp never below 28 and usually around 29 to 30C.
I will say this trip to PG, I noticed a reduction in the vis. maybe due to a plankton bloom or whatever, but there is still some excellent diving to be had there, and don't let the weather or bangka disasters scare you away, I'm more scared riding a trike around up here in La Union.
Enjoy