Charity chiefs in both England and Australia raised the concerns as leaders from the nations and others met at the G7 summit.
Figures from the Government have revealed more than 26 million livestock animals are based across the West Midlands.
The charity claims a zero-tariff deal could mean higher welfare standards of animals farmed in the region become undermined.
Chief executive Chris Sherwood said: "The West Midlands has a proud farm animal welfare record - with thousands of farms rearing millions of livestock to higher legal English standards, and many going above and beyond that.
"But we fear this is all set to be undermined by the signing of a quick trade deal with Australia, which could open the doors in the West Midlands to low welfare imports that undermine our high domestic standards - with beef and lamb a particular concern to us, given Australia's far lower standards.
"People across the West Midlands will be alarmed to know that local supermarket shelves could soon be stocked with agricultural produce reared to lower standards - including mutilations to sheep and growth hormone treatment for beef. This could lead to a lopsided and unlevel playing field for the agricultural community in the West Midlands, and clearly puts farms and hard won welfare standards at risk."
Figures for the West Midlands include 677,000 cattle – including 80,000 beef herd cattle – and almost 2.4 million sheep, with the charity concerned about the important of beef and lamb.
RSPCA Australia CEO Richard added: "Unfortunately, animal welfare standards in Australia are basic at best. In 2021, we still do not have Australia-wide laws that ban the use of sow stalls in pig production, barren battery cages in egg production or require pain relief for very painful procedures like dehorning of calves and mulesing of lambs. Standards are rarely audited and, unless implemented into law, which few are, they are only voluntary.
"The lack of national leadership on animal welfare in Australia needs to be addressed urgently if the lives of farm animals are going to be significantly improved."
To take part in the RSPCA campaign action, visit rspca.org.uk/getinvolved/campaign/farmanimals.