A humpback whale stranded on a Northland beach will remain on the sand for a second night despite the best efforts of rescuers.

The smaller of the two whales stuck on sand in the mid-tide zone a couple of kilometres south of Bayly's Beach died around 7.15am today.

The afternoon attempt to float the larger whale off the beach at high tide failed and would be tried again tomorrow.

All day Sunday and that night, the distressed whales had softly called to each other while lying trapped on the unforgiving sand.

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But as the smaller whale's moans became less frequent then stopped altogether, the larger one displayed anguished behaviour and, as the life left its stranded companion, it slapped its tail up and down on the sand.

''That's the last time it made a sound or moved its tail,'' Project Jonah co-ordinator Daren Grover said a few hours later as the rescue effort focused on the survivor.

Today's effort started at dawn. Two diggers carved a trench the size of an Olympic pool around the whale, with a nearly 3-metre high sandbank.

The plan was to breach the wall when the incoming tide was high enough, filling the trench with water deep enough for the whale to float.

The dream was the whale could get to the water and head straight out to sea.

The animal had been fairly still, occasionally moving its flippers, but as the wall was breached and the incoming tide washed around it, it slapped its tail.

Two hundred people let out a cheer, but if they expected the huge creature to miraculously float then swim down the trench to freedom, they were disappointed.

The water may not have been deep enough, the whale too exhausted or it would not leave the dead one.

Earlier, whale expert Dr Ingrid Visser said the whale's breathing had been irregular at times but otherwise it was in ''pretty good shape''.

There were some concerns the weight of its back muscles and flesh had compressed its organs for too long. Another major worry was that if it was female, it could be pregnant.

The smaller one's death had been a huge blow to the trained rescuers and volunteers who spent Sunday night keeping them both wet, hoping the high tide around 4am would float them into the safety of the sea.

But the tide was ''soft'', not very high, and didn't have the depth or heft to shift the whales.

Everyone referred to the 20-plus metre larger whale as ''she''. Visser, one of the leaders of the rescue attempt which involved iwi, Project Jonah, Department of Conservation and the public, said the behaviour was typical of marine females.

It was also more likely a related younger and older female would travel together.

Behind the stranded whale was a shallow, whale belly-deep channel back into the water.

''See that,'' Visser said. ''She made that when she was coming to help the smaller one. She inched her way toward that one, what that effort cost her we can only imagine, but it looks like the young one got trapped first and she responded to its calls.

''We wouldn't call it a calf,'' Visser said of the smaller one. ''We don't know how they are related but we're pretty sure they are, and they may have been travelling up to the tropics for the breeding season.''

It is uncommon but not unknown for whale species including pygmy sperm whales, to be caught on the beach or their carcasses wash up.

Sonny Nesbit, Te Roroa Trust chairman, said it was the first rescue of humpbacks he had seen on the 100km long beach west of Dargaville.

''Last night was clear and it was a sky full of stars, and the whales were both pretty settled,'' Nesbit said.

It was as if the whales and the helpers were calmly waiting for that tide to change the tragedy around, he said.

Chief executive Snow Tane said Te Roroa was proud of the Dargaville community and the many people on the beach each day who supported the rescue process.

The whales had come ashore right on the boundary between Te Roroa to the north and Te Uri o Hau to the south.

The dead whale was to be dragged up the beach above the tide line tomorrow to be flensed by iwi. Flensing is the removal of bones, organs and blubber.