A gas that was recorded at extremely hazardous levels on a boat where a woman died on Sydney Harbour at the weekend has previously been linked to other deaths at sea and on land.

Hydrogen sulphide is a colourless, toxic gas that is heavier than air and known for its rotten egg smell.

The Lady Rose cruise boat, on which a 39-year-old woman was found dead. Credit:Instagram

It is understood "extremely hazardous" levels of the noxious gas were recorded after 39-year-old Shalina Abdul Hussien was found dead in a toilet cubicle aboard the Lady Rose commercial cruise vessel on Saturday afternoon.

In May last year two men died when they were overcome by hydrogen sulphide at the Ettamogah paper mill, near Albury.