A post on the Al Jazeera website said Mr Hay, cameraman Adil Bradlow and producers Russ Finn and Mohammed Baher were freed yesterday afternoon.
Security officials at Cairo airport said Mr Hay and his colleagues were deported to London, Radio New Zealand reported.
They were forced to leave their equipment behind.
An Al Jazeera spokesperson told RNZ they were not given a reason for their detention.
Al Jazeera thanked all those who helped the group, especially the New Zealand, South African and Irish embassies in the Egyptian capital.
The network said three other employees were still being detained.
Hi all. Landed in London and now resting up after a harrowing few days in Cairo. Thanks to everyone for their kind messages & thoughts
Mr Hay had a cold beer to celebrate his release after arriving safely in London.
Despite his harrowing experience in the Egyptian cells, he said he will continue to cover stories in the Middle East.
"It is daunting and it is a little scary at times, but sure I'm willing to go back because it's one of the big stories, it's a changing story and it's changing all the time," he told Newstalk ZB.
However, Mr Hay said he will be staying away from Egypt for a while.
My thoughts go out to the Al Jazeera staff still detained in #Egypt
Mr Hay's sister-in-law Sandy Eves-Glass said it was brilliant to wake to news of his release today.
She told TVNZ's Breakfast show the last week had been a "nightmare" for Hay's wife Nikki and their children.
Mr Hay had told his wife he and his colleagues had been treated well during their detention, Ms Eves-Glass said.
"They've had food, they've had water - obviously harrowing times, but they are OK."
Mr Hay and his family were going to come back to New Zealand, via a holiday in Bali, Ms Eves-Glass said.
Nikki Hay said it had been a "harrowing" few days.
"To have received that news this morning has been an absolute thrill for us," she told Breakfast from Bali.
She had spoken to her husband this morning, just before he left Syria.
"He was exhausted, but he was sounding absolutely fantastic."
Mrs Hay had not spoken to him yet about his treatment while he was being detained, but contacts at Al Jazeera kept her informed of his condition.
"We knew that he and his colleagues were getting plenty of food, plenty of water, and they were doing OK," she said.
Prime Minister John Key told the programme Mr Hay's detention showed the level of danger some journalists exposed themselves to.
New Zealand's embassy in Egypt would be "very pleased" with the journalist's release, he said.
- Additional reporting by Newstalk ZB
