Nursing mothers should be encouraged to breastfeed exclusively until their baby is six months old, new Australian infant feeding guidelines say.

Although most mothers in Australia start breastfeeding, only a small proportion achieve do so exclusively six months, the National Health and Medical Research Council says.

Only around 25 per cent of women continue to breastfeed for 12 months, say the guidelines, which were released in Canberra on Monday along with new Australian dietary guidelines.

They detail numerous nutrition and health benefits of breastfeeding.

"From a public health perspective, achieving further improvements in both the type and duration of breastfeeding offers considerable benefits for maternal, infant, child, and life-course health for Australians."

The guidelines say health workers have a responsibility to promote breastfeeding first, but if infant formula is needed they need to educate and support parents about formula feeding.

"A mother's informed decision not to breastfeed should be respected.

"When infants are not breastfed, infant formula is the only suitable and safe alternative to meeting their primary nutritional needs," the guidelines say.