The caller said the whale had been lying there doing nothing until then, and now she was powering out to sea.
Department of Conservation staff, Project Jonah volunteers, a marine biologist and members of the public used the incoming tide to try to re-float the whale.
Zainab Aziz, 12, was first to spot the whale.
She said her dad was working on a construction site nearby and alerted them to it.
Zainab said it looked like a really big rock, but then she realised it was the whale and went down and waited with it for help to arrive.
Bid to rescue whale
It's understood the whale - believed to be either a Bryde's whale or an Antarctic Minke - is the same one that was spotted a couple of kilometres up an Auckland river yesterday afternoon.
It appeared to be stranded on a sand bank on the Whau River, close to Pt Chevalier, according to witness photographs and video.
Witnesses at the bank said the whale appeared to be alive and breathing.
Earlier, the Department of Conservation said the whale appeared to be dead.
A DoC marine team was on its way to the whale, and whale protection group Project Jonah was also in the area.
The whale was "still alive at this point", a DoC spokesman said.
A marine team was with the whale, and Project Jonah was also assisting, he said.
"What I understand, is the tide is coming in. The whale is grounded in shallow water and the tide coming in means gradually that water will get deeper."
It was not yet certain that the team would be able to get the whale back into open water once the tide comes in, he said.
"That's an unknown. We don't know what the future holds."
The team would stay with the whale and "do what we can".
Blogger and journalist Russell Brown tweeted from the beach this morning after there reports the whale had died: "Looks like the whale is alive. About to get my feet wet..." He also posted a video of the whale breathing to his Facebook account.
The whale was sighted by a river tour group close to the Riverhead Tavern on the upper Waitemata Harbour.
It appeared to be attempting to catch fish in the shallow waters of the Rangitopuni Creek, a long narrow inlet, The Red Boats Ltd managing director Andrew Somers said.
Mr Somers said seeing the whale yesterday afternoon had been amazing.
"We were departing on our return trip from Riverhead ... we were leaving because the tide was going out and we had to go. I went up front to untie the line and I felt this big wave, and sort of looked around expecting to see a big boat go flying past, and sitting behind us was a big whale," he said.
"It was basically chasing food. It was swimming quickly and going right up on the mud flats and beaching itself, and then it would sort of wiggle itself off again. It did it a couple of times.
"It was a pretty spectacular sight, because it was like having a big ship up there running itself on the bank."
The whale caused quite a stir on board, he said.
"Everyone loved it, they all had their phones out and were yahooing and carrying on."
He added: "It was a good bit of excitement for the day and definitely something to get the passengers talking."