Map from Google - Papua_New_Guinea

The climate in Papua New Guinea is hot and humid throughout the year along the coasts and in the plains, while it progressively becomes cooler, and then colder, as you go up in altitude. In much of the country, covered with dense rainforests, there is not a dry season, so we can speak of equatorial climate, while in some inland valleys and along the south coast, there is a relatively dry season from July to September, and therefore the climate is tropical.
The climate is influenced by the monsoon circulation, the northwest monsoon from December to April, and the southeast monsoon from May to October, which usually bring rainfall in the exposed slopes, but in many areas even in both of them.
Rainfall typically varies from 2,000 to 4,000 millimetres (80 to 160 inches) per year, with some higher peaks on the exposed slopes, where it reaches as high as 7/8 metres (23/26 feet) per year, while it drops to 1,000/1,500 mm (40/60 in) in the southern coast, overlooking the Gulf of Papua and the Coral Sea.

Papua New Guinea

In Madang, on the north-eastern coast of New Guinea, 3,500 mm (138 in) of rain per year fall, with a maximum of 430 mm (17 in) in April and a minimum of 120 mm (4.7 in) in August, so we are in the area that has an equatorial climate. Here is the average precipitation.
Average precipitation - Madang
MadangJanFebMarAprMayJunJulAugSepOctNovDecYear
MadangJFMAMJJASONDY
Prec. (mm)3053003804303852751951201452553403703500
Prec.(in)1211.81516.915.210.87.74.75.71013.414.6137.8
Days17161918171111910111318170

East of the mainland, the islands (including New Ireland, New Britain and Bougainville) are very rainy throughout the year as well. In general, the rains are most abundant from December to April, but not everywhere: in Lae, facing south-east in the Huon Gulf, the rains are most abundant in July and August, when more than 500 mm (20 in) per month fall.
The temperatures are high and stable throughout the year in the north, around 30 °C (86 °F) during the day, while in the south, located farther from the Equator, they decrease a bit in the period that can be called winter, from June to September.
Here are the average temperatures of Madang.
Average temperatures - Madang
MadangJanFebMarAprMayJunJulAugSepOctNovDec
MadangJFMAMJJASOND
Min (°C)232323232322222222222323
Max (°C)303030303030292930303030
Min (°F)737373737372727272727373
Max (°F)868686868686848486868686

The amount of sunshine is usually low or just sufficient in a large part of the country, even poor in mountainous inland regions, where the sky is often cloudy, while along the southern coast, in the dry season from May to October the sun often shines.
The capital Port Moresby is located in the southern area, which as mentioned is less rainy and more sheltered, so much so that it receives only 1,000 mm (40 in) of rain per year, with a relatively dry period from May to November, when rainfall drops below 70 mm (2.8 in) per month; in particular, the rains are scarce from June to October. Here is the average precipitation.
Average precipitation - Port Moresby
Port MoresbyJanFebMarAprMayJunJulAugSepOctNovDecYear
Port MoresbyJFMAMJJASONDY
Prec. (mm)180195170105653530182535501101018
Prec.(in)7.17.76.74.12.61.41.20.711.424.340.1
Days981063432234761

Here, as mentioned, the amount of sunshine is higher than in other areas of the country. In the capital the best period is the one that runs from June to September, because it is also, albeit slightly, the least hot, with maximum temperatures around 29/30 °C (84/86 °F), instead of 31/32 °C (88/90 °F) that are recorded in the rest of the year.
Average temperatures - Port Moresby
Port MoresbyJanFebMarAprMayJunJulAugSepOctNovDec
Port MoresbyJFMAMJJASOND
Min (°C)232222222222212122222223
Max (°C)313131313030293030313232
Min (°F)737272727272707072727273
Max (°F)888888888686848686889090

The sea is always warm enough for swimming throughout the country, although in the southernmost part, where lies the capital, it becomes a bit less warm from July to October, as we can see in the following table.
Sea temperature - Port Moresby
Port MoresbyJanFebMarAprMayJunJulAugSepOctNovDec
Port MoresbyJFMAMJJASOND
Sea (°C)292929292827262526262829
Sea (°F)848484848281797779798284

The interior of the mainland (eastern New Guinea) is crossed by mountains: in the north the Victor Emanuel Range and the Bismarck Mountains, the latter culminating in the 4,509 metres (14,793 feet) high Mount Wilhelm, while in the south we find the Owen Stanley Mountains, whose highest peak is Mount Victoria, 4,038 metres (13,248 feet) high. At 1,500 metres (5,000 feet) the temperature is pleasant: cool at night, around 15 °C (59 °F), and warm during the day, around 25 °C (77 °F). Above 3,500 metres (11,500 ft), the forest leaves room for a sparse mountain vegetation: here the cold is constant and sometimes it may even snow.
From the mountains, several rivers flow, such as Sepik and Fly, which in lowland areas, often at great distance from the mouth, give rise to marshes and swamps, rich in mangroves and gallery forests.

Papua New Guinea can be affected by tropical cyclones, which in the South Pacific occur from November to mid-May, although they are more likely from November to March. The part of the country that is directly affected is the centre-south. The map below is an indication of the areas that have been affected in the past. The northern areas, however, may be partly affected by an increase in wind, clouds and waves.

Papua New Guinea, zone affected by cyclones

The climate of Papua New Guinea is influenced by the so-called ENSO cycle: in the years of El Niño, the rainy season is warmer and drier than normal, and often the monsoon arrives late, while the dry season is cooler than normal in the south; in the years of La Niña, the rainy season from December to April is rainier than normal and can lead to flooding, while the dry season is warmer than normal in the south. During neutral phases (neither El Niño nor La Niña), however, it's more likely for cyclones to form.

When to go

The best time to visit Papua New Guinea, at least in the southern coast which includes Port Moresby and is the only one which experiences a real dry season, runs from June to September, because it is also the coolest. The months of May, October and November along this coast have little rain as well, but they are a bit hotter.
On the contrary, as we have seen the other parts of the country do not experience a real dry season, in fact they are rainy all year round, and some areas, such as the coast where Lae is located, receive the greatest amount of rainfall even in this period. However, the best period to visit the country as a whole is still from June to September, recalling that in many areas there can be showers and thunderstorms, sometimes intense.

What to pack

All year round, tropics-friendly, loose fitting clothing, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
In inland areas, around 2,000 metres (6,500 feet), spring/autumn clothes, sweater and jacket for the evening, umbrella. For the highest peaks, warm clothes, down jacket, gloves, hat, hiking boots.