The summers in Sydney invite long evening admiring the harbour, but did you know that Australia's largest rodent calls the harbour home? As a part of Off Track's summer season we're bringing you the coolest of sounds for summer, and that means we're on our way out to search for water rats.

We meet on a cool morning in the affluent suburb of Mosman, where the only people out on the street are the cleaners arriving for work in big houses, and intrepid ecologists keen on a rat hunt.

We are on the lookout for the water rat, also known as the Rakali.

'It's quite distinct, it's semi-aquatic so it spends a lot of its time in water, and it's filling the same niche as otters,' says Professor Peter Banks from the University of Sydney.

Its scientific name is Hydromys chrysogaster, which refers to its golden belly fluff which you may be privileged to glimpse, if you ever get to see a specimen.

They are known to be elusive and shy, and it's lucky we've got any water rats at all in the Harbour.

When the depression hit in the 1930s the water rat's water-repellent fur was used to substitute for a ban on imported furs.

And back even further, in 1900 when the Bubonic Plague hit the shores of Sydney, a bounty was put on rats. We know that water rats were slaughtered, because one appears in a photo like a trophy laid out at the front of a pile of deceased rodents.

But these guys are the goodies, says Professor Banks, who is investigating whether the large rodents could be used to help control pest-rats.

'Theory tells us that wherever you've got an intact biodiversity, it's going to be robust to invasive species. So if we can make and understand what makes for a robust native fauna, I think we can understand how we can control these [pest] rats.'