The exact size of a frozen colossal squid that was thawed this week has been revealed by New Zealand officials.

The Museum of New Zealand Te Papa Tongarewa says the squid shrank drastically due to water and temperature changes since it was caught accidentally by fisherman in 2007.

After waiting for the tentacles to defrost so the squid could be fully extended, the researchers measured its weight at nearly 495 kilograms and its length at over 4 metres.

The squid's heft and large beak "confirm that it was almost certainly longer and is still the largest invertebrate specimen in the world," says Te Papa spokesperson Jane Keig.

She adds that it also has the largest eye of any animal at about nearly 27 centimetres in diameter.

Some squid, in contrast to the colossal, are just several centimetres long.