Sydneysiders who cook too much food, forget about leftovers and don't plan meals are unknowingly harming the planet and their hip pockets, an environmental expert says.

Bernard Carlon of the NSW Department of Environment and Climate Change says food is the single largest component of household waste in NSW - almost 38 per cent.

"This amounts to an enormous 800,000 tonnes of food waste annually in NSW," he told The Hungry for Change summit in Sydney on Thursday.

"That's 315kg of food waste per household per year that is now disposed of to landfill."

The summit is focusing on developing a sustainable food plan for NSW which might form the basis of future government policy.

Nationally, $5.3 billion of household food waste was thrown away uneaten each year - or about one in every seven shopping bags purchased - but many had no idea of the environmental consequences, Mr Carlon said.

"The majority of people in Sydney are unaware of the impacts of wasting food, underestimate the sheer amount of food that is wasted and waste food in the house simply due to a lack of time, organisation and planning," he said.

"It is of particular concern because across our state, decomposing food and other organic materials is a major cause of greenhouse gas emissions and in this case methane production from landfills."

Mr Carlon said family budgets were also suffering as result, with research showing city households in total throw away more than $1 billion worth of food each year.

"This is made up of $600m worth of fresh food, $182 million worth of leftovers, $131 million of uneaten takeaway food, as well as unfinished drinks and frozen food," he told the summit.

Mr Carlon said people could reduce their carbon footprint by being creative with leftovers, checking the fridge or pantry before shopping, buying only what they need, and not being tempted by supermarket specials.

"I guarantee this will not cost you a cent, it will save money and time, and best of all it will make a significant difference to our planet," he said.

"And it will make a significant difference to our environment here in Sydney and in Australia.