There are over 2000 species of native bees in Australia, some tiny and black as mentioned and others extremely large like carpenter bees. Some are solitary or gregarious and some form colonies (like european honeybees).
As mentioned by grimbeny it is a european honeybee swarm. This is how they sexually reproduce or divide. This is known as an interim cluster. From this cluster the swarm will send out scout bees to look for a suitable nest site. If you can get close to the the swarm, you will notice the bees performing a waggle dance on the surface. This is how they communicate the location of resources or nest sites. Once the bees get excited and the swarm is in agreement it will generate enough heat to lift off by rapidly fanning their wings. Just before lift-off you will here a loud hum and the will drop from the branch and take to the sky. The scout bees then fly above the swarm and basically point in the direction where to go to the new nest site. From the interim cluster stage this generally takes 1-3 days and the lift-off time is usually between 10am - 2pm at this time of year.
As also mentioned, in urban environments this nest site is commonly in vent's, chimney's or wall cavities of houses, or of course in tree hollows. It's personal choice whether you have them destroyed or you let them live and contribute to pollination. They have been feral in Australia for well over 100 years. The competition for nest sites with other animals is a difficult issue. They often nest in cavities with tiny hole entances that no bird or mammal could fit in. Only last week i saw a presentation from a USYD student studying hollows showing cockatoos sharing a tree hollow with a feral hive. The bees stings were unable to penetrate the feathers.
In relation to pollination, it does not come down to native bees being tropical. Whilst that is a factor, the most common place stingless native bees are used to pollinate is in glass or greenhouses so temperature isn't largely the issue. We have succesfully pollinated vegetable crops in Victoria for a seed company using native trigona carbonaria and austroplebia australis. The issue is that these colonial bee species do not forage great distances from their hives and only have the capability of pollinating certain plant species.
We have succesfully used some of the following bee species in certain crops;
Blue banded bees / Teddy bear bees / Carpenter bees - Tomatoes
Trigona and Austroplebia - Macadamia nuts, strawberries, a variety of vegetable crops
Leafcutter bees - Lucerne
Resin bees - limited in some vegetable crops
Cheers Gird
If they make their home in a hollow in one of your trees keep in mind they tend to damage the tree and exasperate the hollow which often leads to failure. We have had to deal with some terrible fallen tree jobs where we have needed to eradicate Bees and it's a pain in the...... where ever they sting you.
I have worked with bees scientifically for over 20 years, and there is no way on earth they can excavate a tree cavity and make a tree fall over. They simply fill a vacant cavity with wax combs. If anything, it would strengthen the cavity. The only time i have seen a limb fall, is when they have chosen to nest in a frail horizontal limb and the added weight of 5-10kg of stored honey has caused it to snap.
Gird
By and large agriculture uses portable domestic hives if needed, feral bees are entirely unnecessary in urban areas and they compete with possums, bats and birds for tree hollows and with native insect species for food.
You would probably assume that however a lot of agricultural pollination in Australia is carried out by "feral European honeybees" very little managed pollination goes on in this country. Farmers are reluctant to pay for bees if the ferals are doing the job for them.
The biggest issue will be if the parasistic mite varroa destructor gets into Australia. It it estimated that all feral bees will be wiped out within 2 years of it's arrival (based on other countries). It is only then farmers will appreciate the necessity for managed bee colonies as pollination of crops will fail without added bees. We are currently the only country in the world without it in our honeybees. This has lead to the formation of a "Pollination Australia" industry alliance in case of a situation like this.
Gird
