A popular Melbourne restaurant chain that asks customers to "pay as you feel" has felt the financial sting of a dip in its patrons' generosity, losing $4000 in one evening.

The summer period is the busiest time of the year for Lentil As Anything, a not-for-profit organisation that operates four vegetarian restaurants in Melbourne and is funded entirely by its customers' donations.

But on a number of occasions, an underwhelming amount of donations has meant the restaurant chain is working at a loss.

After one particularly busy day last week, the Abbotsford branch served almost 1500 people but recorded a loss of roughly $4000.

The restaurant is founded on the motto "everyone deserves a place at the table", and aims to provide a healthy and positive dining experience for everyone – regardless of their bank balance. There are no prices on the menu and customers donate however much money they wish in collection boxes in each restaurant.

Lentil As Anything owner and founder Shanaka Fernando said that overall the restaurant's model was successful, but it experienced some trouble during the summer festival period.

"There seems to be some correlation between the festival-going demographic and the dropping in donations," he said.

Signs at each restaurant remind patrons that a donation of around $12 will cover the cost of a meal and help with restaurant overheads.

Those who are unable to donate much are encouraged to volunteer in the restaurant, or contribute more when they are able to.

Mr Fernando said the Melbourne community had been very supportive and generous since the first restaurant opened in St Kilda in 2000. Just last week a woman donated a portion of land on her 35-hectare farm to growing produce for the restaurant.

But he also said it was hard to ignore the fact that some patrons were treating the restaurant as a place to score a free feed.

Last Monday, the Abbotsford restaurant collected $3995 in donations after serving 1457 people – many of whom had returned from the Rainbow Serpent music festival. It equated to an average of just $2.75 per head.

This led to staff posting a reminder on their Facebook page of the costs of running each restaurant. "We really love seeing the dynamic energy of all the summer travellers and festival goers … But we need your support as well," the post read.

"We are not a spot to get a free feed after some long hours partying on the dance floor."

The organisation lists the total expense of running its Abbotsford site at $24,000 a week, including the cost of staff, rent, food and utility bills.

Across its four Melbourne sites and recently opened site in Sydney, roughly 1 million meals are served each year.

Mr Fernando said this was a "significant operation" and it was important people understood the restaurant did not receive funding or grants.

"Overall we are strong and sustainable," he said "[But] every now and then we get hit by something like this and we just need to remind people that if they think their actions are not impacting on us, then think again."

Despite the significant financial losses in recent weeks, Mr Fernando said the restaurant would stick with the "pay as you feel" model.

"We are trying to put the onus on people to show that people are intelligent and people have got integrity."