A video has shown how workers in Vietnam inject prawns with substances to make them look big and fresh when they are sold and imported, reports Daily Mail. 

The footage shows tiger prawns in a Vietnam factory being injected in the head, tail and middle with a substance to make them heavier for when they are sold. 

Vietnam is the largest source of imported prawns in Australia and they are sold in major supermarkets like Woolworths and Coles.

Mainly black tiger prawns and vannamei are imported to Australia from Vietnam - and they are most often frozen and raw. Australia is known to import ecologically certified black tiger prawns from Vietnam.

The footage, which was shot by a Vietnamese TV station earlier this year, is now going viral on social media with many Facebook users outraged. 

Workers were filmed injected the tiger prawns with a substance known as carboxymethyl cellulose or CMC.

The substance, which is not considered harmful, dissolves in water and becomes a smooth liquid before being pumped into the prawns.   

While carboxymethyl cellulose is often used in food as a thickener for icing, many have labelled the Vietnamese workers as 'shameless' and dishonest for trying to plump up the prawns. 

Vietnam imported about $220 million worth of prawns to Australia in 2014-15, according to figures from Greenpeace. 

It comes after the organisation released a new report last year titled 'Dodgy Prawns' to help customers find prawns sold without slavery, ocean destruction or toxic chemicals.

Greenpeace's prawn guide recommended NSW school prawns caught from estuaries, black tiger and banana prawns farmed sustainably in Queensland, as well as ecologically certified black tiger prawns from Vietnam and eastern king prawns from Moreton Bay, Queensland.

They warned to avoid all imported vannamei, which often comes from Vietnam, due to concerns around mangrove destruction, pollution, and the use of an invasive species.