Map from Google - Tasmania

The climate of Tasmania is oceanic on the coasts, with mild and rainy winters, and cool summers, while in the interior it becomes colder. Being in the southern hemisphere, the seasons are naturally reversed compared to the northern hemisphere, therefore in July and August it's winter.
Tasmania is the southernmost state of Australia. At these latitudes (the island is located between 40 and 43 degrees south latitude) the westerlies prevail throughout the year, so the weather is variable, with a series of disturbances that lead to rainfall especially on the western side of the island. Even the wind blows frequently.
Cold air masses from Antarctica can reach the state most of the year, but they have to travel a great distance above the ocean, so they come a bit mitigated, and are felt mainly in inland areas, where they can lead to snowfall and frost.
On the other hand, short heat waves from the Australian desert can affect the island in the summer (December to February).
The rains are frequent throughout the year, with a maximum in winter and a minimum in summer; however, they vary greatly in quantity, since the bulk of the rain falls on the western side. In fact, precipitaton ranges from 1,500 millimetres (60 inches) per year and more in the west coast, to 2,000 mm (80 in) and more on the western slopes of the inland hills, to about 900/1,000 mm (35/40 in) on the north coast, to 600/700 mm (24/28 in) in the east.
The amount of sunshine in Tasmania is never excellent, because cloudy days can occur throughout the year, however, between a disturbance and the other the sun can come out: on the coast there are on average about 8 hours of sunshine per day in January, and 4 hours in June and July.
Tasmania is an island of 64,400 km² (24,900 sq mi); together with the smaller islands the surface of the state reaches 68,400 km² (26,400 sq mi). Among the smaller islands, stand out King, Flinders and Cape Barren, which are not only the largest ones, but are located apart and to the north, in the Bass Strait, which separates Tasmania from Australia.
Other islands (like Bruny, Maria, Robbins) are a short distance from the main island.

Flinders Island

The northernmost islands (King, Flinders, Deal, Cape Barren) are the ones with the mildest climate, especially in winter. Here are the average temperatures of the most northwestern island, King Island.
Average temperatures - King Island
King IslandJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)1213131110888891011
Max (°C)202120171514131314161719
Min (°F)545555525046464646485052
Max (°F)687068635957555557616366

In King Island, about 900 millimetres (35.5 inches) of rain per year fall; for the northern location there's a marked decrease in the summer rains, which become relatively rare (occurring on average six days per month in January and February). Here is the average rainfall.
Average precipitation - King Island
King IslandJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)3540507010010012511585806050905
Prec. (in)1.41.622.

On the west coast of Tasmania we find Strahan, where winter gets a little cooler because of the more southern latitude. The record low temperatures are about -2/-3 °C (27/28 °F) in the winter months (June to August), and 37/39 °C (99/102 °F) in summer (December to February), recorded during the brief and rare heat waves from the Australian desert.
Average temperatures - Strahan
Min (°C)1111108765667810
Max (°C)212120171413121314161819
Min (°F)525250464543414343454650
Max (°F)707068635755545557616466

The west coast, directly exposed to the west winds, is particularly rainy. The rains are very frequent, but often occur in the form of brief showers, in a context of a variable or unstable weather.
Average precipitation - Strahan
Prec. (mm)8565105120150160180175155125951001510
Prec. (in)

Approximately 40 km (25 mi) east of Strahan we find Queenstown, where rainfall reaches even 2,400 mm (94.5 in) per year, due to the proximity to the hills and mountains of the interior, with a maximum of 270 mm (10.6 in) in July, and a minimum of 100 mm (4 in) in February.
On the east coast we find the capital Hobart, where temperatures are similar to those of the west coast, but the rains are much less abundant.
On the rare lowland towns that are located at some distance from the sea (as is the case in the aforementioned Queenstown) the climate becomes slightly more continental, as you can see from the temperature of Launceston, which is located about 50 km (30 mi) away from the north-east coast.
Average temperatures - Launceston
Min (°C)10109753234679
Max (°C)232321171411111214161921
Min (°F)505048454137363739434548
Max (°F)737370635752525457616670

Launceston gets an average of 680 mm (26.5 in) of rain per year, we are thus in the least rainy area of the state, the eastern one.

The sea in Tasmania is cold throughout the year, however, it reaches 17 °C (63 °F) from January to March in the southern part (see Hobart), and 18 °C (64 °F) in the north.
Sea temperature - Hobart
Sea (°C)171717161514131313131415
Sea (°F)636363615957555555555759

Cradle Mountain-Lake St Clair National Park

As we have just seen, on the coast the temperatures are fairly uniform, although of course they tend to be a bit milder in the north than in the south, while in inland areas they become cooler, both because of the altitude and the distance from the sea. In fact in the interior of the island we find an hilly landscape, and even some mountains exceeding 1,000 metres (3,300 feet), the highest of which are Legges Tor, 1,573 metres (5,161 feet) high, and Ossa, 1,617 metres (5,305 feet) high. It is a pristine area, protected in national parks (such as the Cradle Mountain-Lake St Clair National Park) and UNESCO sites, which includes forests, glacial lakes (such as the lakes Gordon and Pedder), rivers (such as the South Esk River, or the Derwent River, whose estuary is located near Hobart), and waterfalls.
The Highlands are particularly windy, besides they are more exposed to the cold air outbreaks of Antarctic origin, associated with snowfall and frost, which may occur occasionally even in summer.

In the Central Highlands, at hill elevations the winter is cold enough, and during cold spells the temperature can drop to -10 °C (14 °F) and even below. Here for example, the average temperatures of Tarraleah, situated 600 metres (2,000 feet) above sea level.
Average temperatures - Tarraleah
Min (°C)776420001346
Max (°C)191917141188911141618
Min (°F)454543393632323234373943
Max (°F)666663575246464852576164

Precipitation in Tarraleah amounts to 1,100 mm (43 in) per year, with a minimum between summer and early autumn (60/70 mm or 2.4/2.8 inches per month between January and March), and a maximum in winter (110/120 mm or 4/4.7 per month in July and August).

Above a thousand metres (3,300 feet) the climate becomes very cool or cold all year round. For example, in Mount Wellington, 1,271 metres (4,170 feet) high and located near Hobart, the average temperature in July and August approaches the freezing point. The cold records are around -7/-9 °C (16/19 °F); it is however rare for the temperature to remain below freezing even during the day. Even in summer, sometimes at night the temperature can go down to around 0 °C (32 °F) or even slightly below.
Average temperatures - Mt Wellington
Mt WellingtonJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)56421-1-2-2-1023
Max (°C)1314118642347911
Min (°F)414339363430282830323637
Max (°F)555752464339363739454852

Precipitation in Mt Wellington amounts to 960 mm (37.7 in) per year, and is well distributed throughout the year.

When to go

The best period to visit Tasmania is the summer, from December to February, being the warmest season, when the days are longer and the rains less frequent, and this is an advantage for the inland and mountainous areas; it is true that sometimes you may experience a very hot weather, which is felt more at low altitude: in these cases, however, the heat is quite bearable because of wind and low air humidity.
Considering that even in the height of summer it can be a bit cold at night (lows can drop below 10 °C or 50 °F even on the coasts), it's better to bring a light jacket and a sweater even in this period, and also a heavier jacket if you plan visit the inland and mountainous areas, where during the day the wind blows more easily and at night the temperature can drop to around freezing. Also a raincoat may be helpful for the rain showers.