Keen burger lovers have been lining up since 4am for a taste of U.S. chain In-N-Out's famous burgers, and staff started turning people away even before the pop-up opened.

Callan James, 22, and Ella Baldwin, 26, drove from Newcastle at 8am to stand in line and their persistance paid off as they were two of the lucky hundreds to get wrist bands.

'We drove two hours. We just had the day off and we've had them in America before and they're delicious,' Mr James told Daily Mail Australia.

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Happy-looking customers sit down for their In-N-Out Burger at Dead Ringer in Sydney on Wednesday

Customers wait in line at the In-N-Out Burger pop-up restaurant to be served.

Hundreds of people lined up for the opportunity to eat In-N-Out - but only 300 were there on time to get a burger

The large line would around street corners. Some of those in line arrived as early as 4am to get a taste of In-N-Out

An employee at the In-N-Out pop-up prepares drinks for the waiting customers on Wednesday

2Day radio host and former Bachelorette star Sam Frost attended, giving customers money to pay for their meals

An In-N-Out customer poses for a photo with Sam Frost, who was handing out cash to pay for people's meals

In-N-Out staff were letting people into the restaurant in groups in order to avoid mayhem at the venue in Surry Hills

In-N-Out staff behind the counter at the pop-up restaurant, which served enough burgers for 300 people

Burgers being delivered to a customer at the pop-up In-N-Out Burger, which opened about 12pm on Wednesday

About 12pm, In-N-Out fans began to post pictures of their meals from the American fast food joint

Those who had lined up early enough to receive burgers took to social media to post pictures of the sought-after fast food.

In-N-Out fans posted photos of their purchases to social media on Wednesday. Some had travelled from as far away as Newcastle for their burger fix

The 'Double Double' burger was being sold for $5 at the pop-up restaurant in Surry Hilss

A menu at the pop-up shows the pricing for the items - including $5 Double-Double burgers and $3 hamburgers

Sydney's Howard Strutt, 17, was in line at 10am and had even been offered money to buy burgers for a stranger.

'He was going to give me $100 to get six burgers,' he said.

Howard, who received a wrist band, said he did not want to take any chances on missing out like the time he lined up for the wildly popular Game of Thrones exhibition.

'I got cut off when the [exhibition's] line was very long and I didn't want it to happen like at the Game of Thrones exhibition,' he said.

'I waited four hours from 8am.'

Callan James and Ella Baldwin drove from Newcastle on Wednesday morning to get their burger fix

Theo Zacharopoulos was the last person in the long line to be given a wristband allowing him to purchase a burger

Howard Strutt and William Alexander. Howard, 17, lined up by 10am and even had a stranger offer him money to buy him a burger

A huge queue had formed well before 12pm, when the pop-up opened, and some fans were turned away as there were not enough burgers to go around

Just some of the burger fans lining up to get their hands on an In-N-Out Burger in Sydney on Wednesday

Posts on social media suggested there were hundreds of people in the line by 11am

Instagram user Antspats posted this picture of the In-N-Out Burger pop-up in Surry Hill about 11am on Wednesday, saying they thought all 300 wristbands, thought to be required for service, had been handed out to people in the line

Outside the In-N-Out Burger pop-up on Bourke Street, Surry Hills, Sydney on Wednesday

In-N-Out fans at the pop-up restaurant on Wednesday, which served enough burgers for about 300 people, according to reports

The Instagram user who posted this picture added the caption 'so near yet so far #justmissedout'

TodayFM breakfast presenter and former Bachelorette star Sam Frost (centre) paid for the meals of everyone who ate at In-N-Out on Wednesday out of her radio station's bank account, according to her program's Instagram account

The burger joint, which last made an appearance in Sydney in January last 2015, planned a return to the city for only four hours on Wednesday afternoon, with only about 300 burgers available, according to reports.

Just after 11am, fans lining up began reporting that people in the hundreds-long line were being turned away before the pop-up even opened, as there were not enough burgers to go around.

Those who did manage to get their hands got an extra surprise, as TodayFM breakfast presenter and former Bachelorette star Sam Frost even popped by the line to giveaway $10 to patrons who braved the 37-degree heat to satisfy their cravings.

Dressed in a baby pink, sleeveless top and jeans, Frost happily posed up with fans as she handed out the blue notes.

On the radio station's Instagram, they said they had paid for everyone in line.

Theo Zacharopoulous, 36, was the last person to be handed a wrist band, much to his relief, and said he would have cried if the outcome had been different.

'I got here 20 minutes to 11am. We heard people were waiting from 4am,' he told Daily Mail Australia.

'I've come from work and my boss has been ringing asking: "When are you coming back?"

'Every year for five years, I've missed out.

'My sister told me about it this time. It's my lucky year.

'I probably would have cried if I had missed out, probably would have run to the car and cried.'

Two pleased-looking diners get their burgers from a server on Wednesday

Staff at the In-N-Out Burger pop-up on Wednesday morning before opening

The inside of the In-N-Out Burger pop-up looked far quieter than the busy line of hundreds of people outside

Fans appeared to be relishing their meals after - for some - what was a long wait in line

A close-up of one of the In-N-Out burgers gives those who missed out a picture of what they missed out on

Those who were lucky enough to be in line early enough to get a burger were given wristbands

About 300 of the wristbands were given out to avoid confusion in the line up for the burgers

To accommodate the frenzy, staff are letting groups of 10 in at a time.

The first customers began to be served just before noon, and the gleeful recipients took to social media to post images of their meals.

A report posted to The Fatties burger appreciation society page on Facebook - with more than 35,000 members - said the In-N-Out burgers were being sold for $5, and gave them a rating of 3.9 out of five 'pickles' - the group's equivalent of stars.

Fans were going crazy on social media and at the venue before opening, with lines for the burgers, fries and secret menu items - including 'animal style' toppings - starting to form as early as 8am ahead of the 12pm opening.

Confirmation of the In-N-Out pop-up taking place was only made by the restaurant hosting the event on Tuesday, but it did not take long for excitement to build.

News spread quickly online as the word got out and social media posts about the pop-up attracted thousands of 'likes', comments and shares as the opening hour drew closer.

People took to the internet to announce their joy or lamented being stuck at work and missing out.

The pop-up is located in the Dead Ringer restaurant at 413 Bourke Street, Surry Hills, Sydney

In-N-Out will be available at Dead Ringer Sydney (location shown above) for four hours only on Wednesday from 12-4pm

Social media posts about the pop-up attracted thousands of 'likes' and comments, as people announced their excitement or lamented being stuck at work and missing out

The burger joint has a cult following for many reasons - including its 'secret' menu and options such as 'animal style' toppings on burgers and fries

Confirmation of the In-N-Out pop-up taking place was only made by the restaurant hosting the event on Tuesday

Others were annoyed the pop-up was returning to Sydney instead of heading to a different location, such as Melbourne.

One man even begged someone to take him In-N-Out at working, saying it would 'make his year'.

Previously, when the burger chain has operated pop-ups in Sydney in Parramatta and Darlinghurst, the burgers have been munched in a flash and long queues formed outside venues selling them.

In-N-Out's first restaurant opened in 1984 in California and the chain is known for refusing to use frozen food, fair wages and celebrity endorsements, Broadsheet reported.

Those who swear by the burger joint say its lack of freezers - meaning fresh, recently prepared ingredients, and features such as the not-so-secret 'secret menu' are just some of the reasons In-N-Out is so good.

The pop-up was held at the Surry Hills restaurant Dead Ringer.

Social media users posted these pictures of wristbands being given to those in the line to get In-N-Out in Sydney

The pop-up will be the third time In-N-Out has come to Sydney within the past three years

In-N-Out has appeared in Sydney before, operating pop-ups in Parramatta and Darlinghurst

Burger fans were both delighted and despairing at the news - depending on whether they would be able to make it in time for a burger

Between 300-500 burger fans will be getting In-N-Out, as there are reportedly a limited number of the burgers available

Reports said people were lining up for the burgers as early as 8am on Wednesday morning

The news of the burger chain coming to Sydney spread quickly on social media

This man hoped for someone to take In-N-Out to him at work, saying it would 'make his year'

Other burger fans were disappointed the pop-up was not going to Melbourne

Burger fans who were missing out on the four hour burger frenzy lamented not being able to get their hands on In-N-Out

Those wanting to grab an order from the pop-up needed to be in early, as fans were lining up before 9am

Social media has been abuzz with news of In-N-Out returning to Sydney - if only for a few hours on Wednesday afternoon

A image shared by an In-N-Out fan the last time a pop-up sold the burgers in Sydney