Southern Water said the pipe burst in Chestnut Avenue had affected homes in the SO53 and SO16 postcodes leaving them with no water or low pressure.
The company said supplies would gradually return to normal later as the main was being repaired.
Chestnut Avenue is expected to remain closed until Friday as the burst had caused substantial damage.
Engineers have closed off the road between the Bournemouth Road junction and the Tollgate junction.

Bethany Haines also told ITV News that she hoped her father's body would be returned to his family.
Mr Haines, who was born in Yorkshire and educated in Perth, was killed by IS earlier this month.
He had been taken hostage while carrying out aid work with refugees in Syria.
Mr Haines had been living in Croatia with his second wife, who is Croatian, and their four-year-old daughter. His parents live in Ayr.
His older daughter Bethany gave her television interview from her home in Scotland as MPs prepare to vote on military action against IS.
The 17-year-old said: "IS need to be eradicated. They can't continue this way. They can't be doing this to people and get away with it. No matter what nationality, if they are Western or not.
"Hundreds of Syrians have been killed by them. They need to be stopped. If airstrikes and ground force is what it takes, that's what it takes."
Ms Haines said her family had been touched and moved by the messages of support they had received.
She added: "Anyone who needed help he was there. Tragically he's lost his life doing what he loved but he wouldn't have stopped. Even if he'd come back after this he would've gone out again and again and again. He loved helping people.
"He was such a bright character. Always making people laugh, telling jokes. He was a brilliant man. And he will be remembered like that by everyone who knew him and probably by people who didn't know him.
"He will be remembered and not for the way he was shown in that video. But for the truly amazing work he did and the bright character he was."
Bethany Haines said her father's commitment to helping others had encouraged her to follow his example.
"I'm hoping to do some aid work after next year and do a bit of work with a charity," she said.
"Hopefully it will spark a career doing that. He'd tell me of all the people he'd helped and I want to do that too."

The 26-year-old was released by York City after failing to score in 14 appearances last season.
However, he netted 26 times in 69 appearances in a two-season spell at Barnet between 2012 and 2014.
Hyde is the Boro's fourth signing of the summer, following left-back Andrew Fox and forwards Matt Godden and Rowan Liburd.
"I know this league inside out now and any team can go on a run, but it's who does it for the longest period that counts.
"It's about winning games and fingers crossed I can help Stevenage do that this season," he told the club website.
Details of his contract with Stevenage have not been disclosed.
Find all the latest football transfers on our dedicated page.

The ship took shelter near Mounts Bay after an initial rescue at 22:00 BST on Sunday failed due to poor weather conditions, the coastguard said.
The men were airlifted to Royal Cornwall Hospital.
The Panamanian registered Bochem Oslo vessel had the highly corrosive acid on board for maintenance reasons. No cargo was on board.
Commander Scott Armstrong, who was flying a helicopter from RNAS Culdrose, said the swell 20 miles (32 km) off St Ives was a problem.
"We estimated there was about a 40ft movement on the ship, it was just too much, it was moving around too quickly," he said.
It is understood the tanker was en-route to Germany to pick up cargo.
A burst water main in Eastleigh has caused flooding in a road and left about 4,000 homes without water.
[NEXT_CONCEPT]
The daughter of David Haines, who was killed by Islamist militants, has said the Islamic State (IS) group should be "eradicated".
[NEXT_CONCEPT]
League Two side Stevenage have signed their third striker of the summer by bringing in free agent Jake Hyde.
[NEXT_CONCEPT]
Three men have suffered burns from nitric acid while on board a ship off Cornwall.