New standards that come into effect in March set out minimum access to daylight, functional space and natural ventilation. They are meant to reverse the trend of smaller or micro apartments that have seen homes as small as 11.2 square metres, smaller than the smallest new Jayco caravan in size.

The median size of a new one-bedder in inner Melbourne dropped from 55 square metres in 2013 to 44 square metres in 2016, according to Oliver Hume Real Estate Group.

Big four banks have already made a policy in line with the new standards of refusing to lend for one-bedroom apartments smaller than 50 square metres.

Developers in recent years have been making planning applications for larger apartments, with the proposed new guidelines in mind.

However, a source told The Age that most of the apartment projects due to be completed in 2017 were designed and sold in 2012 or prior, before the discussions about the new guidelines had been made public.

"Those apartments purchased off-the-plan that are less than a certain size will find it a challenge to find financiers," the source said.

Lenders are mostly concerned about the resale value of apartments that do not meet the minimum standards.

The managing director of mortgage broker Homeloanexperts, Otto Dargan, said for small apartments, buyers may have to up their deposit to 20 per cent or more.

That could leave some buyers scrambling for cash at settlement time to plug the gap.

He said even though banks often provided customers with a "pre-approval," this could end up being worthless, as lenders had specific restrictions on factors such as a unit's location or its size.

"Before committing to purchasing an apartment, people should check the property with their mortgage broker or bank manager to make sure it meets the lenders' criteria," Mr Dargan said.

The warning comes as a Melbourne buyer is caught up in a blame-game with NAB about a St Kilda apartment purchase that went wrong, leaving him $40,000 or more out of pocket.

 First home buyer, 20-year-old Alexander Tashevski-Beckwith, claimed he had verbal approval from a bank manager to sign the purchase contract, but the manager then mysteriously left the company before sending out the final loan approval documents.

He was told the apartment was too small, despite being told that the size was not an issue previously, he claimed. It left him without finance at settlement time.

Mr Tashevski-Beckwith has produced witnesses, such as the vendor's agent and a conveyancing solicitor, to confirm his side of the story.

His mother, Marika Tashevska, said she and her husband were scrambling trying to raise the $214,000 from family members to settle on the 47-square metre apartment so he didn't lose his money.

The case is further evidence that banks have responded to directions from APRA to be more prudent about loans for apartments, particularly those considered high-risk.

A spokeswoman from NAB said they had been trying to meet with the family to work out what went wrong and find a solution. It had no documents to show a loan had been approved.

She said the bank had to be responsible in evaluating home loan applications and economic risks and that properties of less than 50 square metres are generally not approved for lending purposes.

"We are prepared to offer compensation for any financial losses if it is the case that NAB has been at fault and caused these losses in this instance," she said.

There were approximately 122,500 apartments completed across Melbourne in 2015, the latest annual figures available, according to the Charter Keck Cramer database.

One-bedroom apartments were mostly sized between 41-60 square metres, within a price range of $435-454,000, according to the database.

-with Clancy Yates