New Zealanders will get a once-in-a-lifetime chance to see the Transit of Venus on Wednesday - weather permitting.

Transits of Venus - when the Solar System's second planet travels between Earth and the Sun - are rare and predictable astronomical events, with pairs of transits eight years apart separated by long gaps of more than 100 years.

Astronomer Claire Bretherton, from Wellington's Carter Observatory, says New Zealanders should at least try to glimpse the event, their only chance in their lifetime.

The last one, in 2004, was not visible here and the next one will not be for another 105 years.

The transit was in the past used to measure the distance between Earth and the Sun.

Scientifically the transit is not so important anymore, but it is historically significant as the transit of 1769 brought Captain James Cook to New Zealand, she said.

However, the weather may prevent anyone here seeing the transit. Heavy rain is forecast for much of the country, and snow in the South Island. However, it may fine up in the south.

"In order to see the transit you need to be able to see the sun. It's not looking great," Dr Bretherton told NZ Newswire.

A number of websites would be streaming the transit visible from other points around the world, so people could go online to see it, she said.

The first contact will be at 10.15am, when Venus first touches the edge of the Sun, and it will then move across the Sun until 4.43pm.

The transit is being used as a symbolic marker for the Transit of Venus Forum, where policy makers, journalists, educators, business people, community leaders will gather in Tolaga Bay to witness the transit together with locals, followed by a two-day forum in Gisborne discussing improving the quality of life through science.