A newly discovered asteroid the size of a whale will fly by New Zealand tomorrow morning.

Nicknamed Pitbull, the giant chunk of rock was spotted by scientists for the first time a week ago and is thought to be as much as 4.5 billion years old.

Nasa says it will be a close shave but there is no risk. At its closest approach, expected at 6.18am tomorrow, Pitbull - named for its small size threatening path - should be visible over New Zealand.

The celestial visitor, officially known as 2014 RC, is expected to pass 40,000km from Earth - one-tenth of the distance to the moon.

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Stardome Observatory astronomy educator David Britten said spotting it would be difficult - it was small, faint and would be moving quickly.

But it was a rare and exciting opportunity for stargazers.

An asteroid visit of this nature happened only once every year or two and it was not known whether Pitbull would visit again.

"Earth's gravity will have altered its orbit," Britten said.

Asked what excited him most about the asteroid, Britten said: "It's going to miss us."

Last year's Chelyabinsk meteor, which damaged thousands of buildings in Russia and injured 1500 people, was about the same size, he said.

"While this celestial object does not appear to pose any threat to Earth or satellites, its close approach creates a unique opportunity for researchers to observe and learn more about asteroids," Nasa said.

National Geographic said people would need telescopes with at least medium-sized (15 to 25cm) mirrors to view the asteroid.

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Asteroids and comets are understood to be remnants from a giant cloud of gas and dust that condensed to create the sun, planets, and moons about 4.5 billion years ago.

Most asteroids are in orbit between Jupiter and Mars.

Nasa said asteroids ranged in diameter from just a few metres to the size of Ceres, which is 948km wide.