Birthday sponge cakes sold at Woolworths and lamb chops prepared at Coles were among thousands of products found to be weighing less than what was promised on packaging, according to the National Measurement Institute.

In 2014-15, Australia's peak weights and measures body issued 3962 non-compliance notices to traders, up 13 per cent on the previous year's figure. It sent 139 warning letters and imposed 98 fines totalling $92,650.

The Aldi threat has forced supermarkets to cut their prices. Credit:Quinn Rooney

Nearly half of the breaches related to underweight or incorrectly labelled packaged products.

For the first time, traders were prosecuted under the National Measurement Act, resulting in four convictions, with two against the supermarket giants.