NFU Scotland (NFUS) found in a snapshop survey only about half the volume of imported lamb on the supermarket giants' shelves from outside the UK compared to 100% Scottish or British lamb in Morrisons, Marks & Spencer and Sainsbury's.
The farmers' union said it was a major disappointment, adding: "We are calling on these retailers to get behind the campaign and give Scottish shoppers the best chance of buying more Scottish lamb."
Last week a Scotch Lamb campaign - Wham, Bam, Thank You Lamb - was launched by Quality Meat Scotland after it emerged a typical Scots household only buys lamb six times a year.
NFUS President Nigel Miller, who joined staff in visiting supermarkets in the Lothians and Borders, said: "It was a huge disappointment to find around half of shelf space in Tesco and Asda being taken up by New Zealand or Australian imports, even after the campaign launch.
"Given that there are ample supplies of great Scottish lamb coming forward from farms, we will be urging Tesco and Asda to get right behind the campaign and dedicate more space to home-produced product on their shelves. We hope, when we revisit these stores in the weeks ahead, significantly more packs of Scottish lamb will be filling the shelves."
NFUS say at the Tesco store at Galashiels, they found 83 packs of lamb with 39 packs from New Zealand and 13 of lamb mince containing mixed New Zealand and British lamb.
Asda in Dalkeith had 49 packs of lamb on the shelves, 21 of which contained Australian or New Zealand lamb.
An Asda spokeswoman said: "Our policy is to source 100% Scottish lamb when it's in season and we are currently investigating as to why this wasn't available at the time of the NFU visit."
A Tesco spokesman said: "We are committed to offering our customers the best local and regional produce Scotland has to offer.
"In our stores in Scotland we offer 100% Scotch lamb on our counters all year round."