The high number of migrants landed in Sicily and the southern Italian mainland between early Wednesday and Thursday afternoon followed the arrival of just over 2,500 over the weekend.
The numbers reflect a pattern of boats timing their departures, mostly from Libya, to coincide with lulls in the often adverse weather conditions at this time of year, experts said.
Over the summer, there was a steadier flow of migrants arriving in Italian waters with more than 150,000 having been picked up by the navy or coastguard in the last year.
The latest figures have doused expectations that the numbers would fall off with the approach of winter and as a result of Italy's suspension of its naval search-and-rescue operation, Mare Nostrum, which some critics had argued was encouraging migrants to take the risk of setting off in what are almost invariably overcrowded and unsafe vessels.
"We always see a slowdown when the weather is inclement but as soon as the conditions are appropriate they leave in bulk," said Martin Xuereb, a former chief of defence staff in Malta who is now director of Moas, a private rescue operation based on the island.
"That is what happened over the weekend - there was a window of opportunity and 2,500 left at one go," he added.
An added danger
"From now until February we will see this - people leaving in bulk and that makes it even more dangerous. Rather than dealing with two boats at a time, there is a possibility of having to deal with 12 to 15 boats at once.
"The weather in the Mediterranean in the winter can change very, very quickly. People think 'it's not the Atlantic' but the Mediterranean at this time of year can be very, very unforgiving.
"We have some terrible disasters in the last year and I think it is possible that we could see something even worse because of the risk of having multiple distressed boats all at one time and the available (rescue) assets will be spread very thinly," he said.
"That really is an added danger because of the suspension of Mare Nostrum," he added.
The latest arrivals reflected recent trends with the biggest contingents coming from Syria and Eritrea, followed by Afghanistan.
Many boats are abandoned by their crew of people smugglers before being accompanied to port or evacuated by the coastguard. In one case on Thursday three suspected traffickers were arrested. Unusually, they were Ukrainian nationals.
Xuereb said the new pattern showed that the European Union, which has replaced Mare Nostrum with a much more limited operation mandated to patrol waters up to 30 nautical miles off its southern coastline, needs to change policy.
"People are going to keep coming. The war in Syria is ongoing and the problems of Eritrea are not going away," he told AFP.
"Tankers and other merchant ships will help (with rescues). They have a legal obligation to and most of them are happy to do that. But they are not equipped for saving lives," he said.
"Whatever is in place should be designed to mitigate against the loss of life at sea across the whole Mediterranean."