The 18-year-old Tarrawanna man was born with a rare genetic disorder called phenylketonuria, or PKU, which means his body can only tolerate six grams of protein per day.

For most of his life a federal government grant has helped his family cope with the cost of the medical, low-protein products that make up the bulk of his diet - but a budget decision will change all that.

Callum, and other Australians born with different types of Inborn Error of Metabolism (IEM), have discovered in the fineprint of the May budget that the government will stop the $256.75 monthly grant to sufferers by the end of the year.

While the supplements Callum needs are covered under the PBS, the food is not. With a loaf of bread for someone with PKU costing $10; a box of cereal $14.65 and a small block of cheese $28, the grocery bills can be sky high.

"I think this decision is very unfair as these foods are our staples - we have no alternative," he said. "It will be very difficult for families to afford to continue to buy these specialised products, especially families like mine where more than one child has the condition."

Callum's brother Timothy and sister-in-law Bianca have the same metabolic disorder, and due to its recessive nature any children they have will also be affected.

"These low-protein medical foods make up 75 per cent of our daily diet," Bianca Buecher, 25, said.

"The increased range of medical protein supplements available on the PBS are not substitutes for these medical foods.

"Successful management of IEMs require both the low protein diet and the medical protein supplement.

"Without the grant, we will have to bear the financial strain of purchasing these essential products - and if we have children that will be multiplied.

"If we don't comply with the diet, short-term effects include fogginess while the long-term ramifications are mental retardation."

Bulli resident Kate Kline also contacted the Mercury about the government's decision to cut the grant. Her two Maroubra-based nephews - Josh, 8, and Tom, 6 - also have PKU.

"Every child with PKU is different - my nephews can tolerate 14 grams of protein a day but others with the condition may only be able to have 3 grams," she said.

"These specialised foods have to be imported, so my sister Micaela Hopkins also has to pay the delivery costs too.

"If families do not receive the grants, it will be very hard for them to afford to keep their children on these diets."

Cunningham MP Sharon Bird said the proposed cuts by the federal government were "unfair and mean".

"We're not talking a huge bucket of money - there's only 904 families in Australia who receive this monthly grant," Ms Bird said. She said it is not a huge cost but it makes a huge difference and the families were not consulted about the changes, and only learnt about it post-budget.

In response to questions about the grant cut by Opposition leader Bill Shorten in parliament this week, Prime Minister Tony Abbott said there had been a number of developments since it was introduced in 2001.

"First of all, there are now far more regular supermarket items like cornflour that can replace wheat flour, and rice milk that can replace dairy milk, for people managing protein disorders," Mr Abbott said.

However, Ms Kline said it was just not possible to create a diet for someone on a low protein diet from supermarket foods alone.

The Metabolic Dietary Disorders Association is now calling on the Federal Government to "urgently reconsider their decision".