Auckland Zoo is down to one blue penguin after a series of deaths that has baffled keepers.

Four of the zoo's six penguins have died of a mysterious ailment in the past six months.

A fifth penguin drowned in a bucket while trying to escape from an emergency enclosure keepers made to keep it safe.

Zoo veterinarian John Potter said the zoo would like more penguins to replace Ani, Henry, Lucy, Pluto and MacGonagal.

The last penguin, Coral, was lonely, and the enclosure would be improved in an effort to make her and her new companions more comfortable.

But he said staff were "stumped" about what had happened to the other penguins. Tests of their swimming water had not found any possible cause of the problem.

The trouble started in April when keepers noticed the penguins had unusually oily coats and did not seem to want to swim.

Too much oil makes penguins' coats absorb water and they can risk freezing to death.

It is not known if they were staying out of the water because of the oil in their coats or because there was something wrong with the water in their enclosure.

Over the next six months, the penguins were hand-fed and washed to try to encourage them back in the water. But four died, each of a different ailment.

"We tend to think it was probably because the immune systems of these birds were affected," said Mr Potter.

The birds had been in the same enclosure since 2002 with no problems - it was possible there had been a problem with the water that only birds could identify.

In December, puzzled keepers moved the last two penguins to a small enclosure so they could to keep a closer watch on them.

It was there that the fifth penguin drowned, after it fell into a bucket of water that had been used to prop up its enclosure during an escape attempt.

"It was just one of those things you wouldn't think about in advance - a penguin drowning," said Mr Potter.

Blue penguins are not endangered in the wild and Mr Potter said the zoo hoped to get more from animal rescue centres once the enclosure had been upgraded.

They would be birds that had suffered an injury that restricted their ability to feed in the wild.