Buying an apartment in some Brisbane suburbs will soon become a lot more difficult after major banks released lists of suburbs deemed risky, where they will require a deposit of up to 20 per cent for a home loan.

Some banks have already imposed the restrictions, but from Monday Suncorp is set to red flag almost 40 locations.

It comes after the inner-Brisbane unit glut saw prices slump to their lowest in three years, with a recovery not expected for at least 12 months.

Top 10 Brisbane postcodes requiring 20pc deposit: 4000 Inner Brisbane 4010 Albion 4006 Fortitude Valley, Bowen Hills, Newstead, Herston 4101 Highgate Hill, South Brisbane, West End 4102 Dutton Park, Woolloongabba 4005 New Farm, Teneriffe 4011 Clayfield, Hendra 4032 Chermside 4122 Mansfield, Mount Gravatt, Wishart 4171 Balmoral, Bulimba, Hawthorne

At least four major banks are refusing to loan more than 80 per cent of the cost of a unit in Brisbane's CBD, Fortitude Valley and the inner-north suburbs of Bowen Hills, Newstead, Herston and Albion, citing problems such as in influx of new apartments and "weakness in the investment market".

Fortitude Valley mortgage broker Caroline Jean-Baptiste said the changes were impacting first home buyers and people looking to downsize, such as divorcees.

"Where the apartments are going up, that's where lenders are considering a higher risk area," she said.

"It does make getting into the market much more difficult, but sometimes you need to find a more creative solution."

Ms Jean-Baptiste said it was mortgage insurers rather than the banks themselves demanding the restrictions.

"Quite often mortgage insurers will not insure in high density postcodes where the risk is higher," she said.

"Every bank that uses a certain mortgage insurer will be affected by this lender's mortgage policy."

The ABC has obtained data from four major banks revealing suburbs where loans will be more difficult to secure:

Westpac* Suncorp ANZ National Australia Bank 4000 4000 4000 4000 4005 4005 4006 4006 4006 4006 4010 4010 4122 4007 4011 4032 4010 4009 4014 4101 4101 4010 4102 4102 4011 4171 4215 + 32 more

postcodes * Westpac more commonly assesses mortgage applications based on the apartment block rather than the postcodes. Postcodes here indicate areas where Westpac has restrictions on apartment blocks.

Finance and mortgage broker Julie Bammon said any "high-risk" borrower would be affected by the growing list of red flagged postcodes.

But she said not all banks would turn down a loan in high-risk areas.

"There would most likely be a bank or a lender where that borrower would be able to fit because different lenders concentrate on different things," she said.

In a statement, a spokeswoman for the National Australia Bank said its move to red flag more suburbs was necessary for the bank to remain a responsible lender.

"We still lend across all areas, however in some geographic regions we may apply a lower maximum loan-to-value ratio on applications," she said.

Inner-city Brisbane residences are being viewed warily by the banks. ( 612 ABC Brisbane: Jessica Hinchliffe )

"This is the right thing to do for our customers, for our business, and for the Australian property market."

A Westpac Bank spokeswoman said they worked slightly differently, assessing whether to mortgage a unit based on the building rather than the suburb or postcode.

"As a prudent lender, we have controls in place to prevent us becoming overly concentrated in a single address, however we don't auto-decline in these circumstances," she said.