Mortgages: The Commonwealth Bank has to catch up in the home loan market. Source: News Limited

New data reveals that the mortgage book of Australia's biggest home lender grew by 0.17 per cent in January -- less than half the pace of all the other major banks.

The data, released yesterday by the banking regulator, lays bare the reason for the CBA's combative reaction to National Australia Bank's campaign to poach mortgage customers.

It shows that NAB's mortgage book grew almost 1 per cent in January -- more than fivefold the growth rate chalked up by the CBA.

Including figures from the CBA's cut-price subsidiary Bankwest, the group still only fattened its mortgage book by 0.25 per cent in January, to $291.6 billion.

The figures come as the banks seek mortgage growth in lieu of the uptick in business lending that has been widely forecast but yet to materialise.

Nomura analyst Victor German said the data showed that CBA management, which has signalled its intention to recapture more market share, "still have a bit of a way (to go) to catch up".

Last Friday, the CBA fired the biggest salvo yet in the mortgage wars that were unleashed by NAB earlier last month with its offer to pay exit fees for customers who jumped ship from its biggest rivals.

The CBA released a new home loan product that carries no fees and -- with an interest rate of 7.24 per cent -- is priced significantly below the standard variable rates of all the other major banks.

Credit Suisse analyst Jarrod Martin said: "Loan growth across the system has been quite subdued.

"For a period of time, both Westpac and CBA were willing to cede some form of market share. But now, with lending still subdued, they're looking at defending their positions."

The publication of the Australian Prudential Regulation Authority lending data coincided with the release of downbeat customer satisfaction figures for CBA.

Polling by Roy Morgan Research indicates the CBA is paying a heavy price for aggressively lifting its standard variable mortgage rate on Melbourne Cup Day.

Roy Morgan found that the CBA has since suffered the biggest fall in customer satisfaction among the major banks -- a 2.6 percentage point slide over the three months to January, to 72.7 per cent.

While it is still leading NAB, on 71.8 per cent, the gap has narrowed significantly. ANZ leads the big four on 75.4 per cent.

Separately yesterday, APRA provided further clarity on the rules it will enforce in Australia as part of a new global accord designed to strengthen the balance sheets of banks.

Cash and government securities are among a select group of so called "level one" assets banks will be required to hold in what analysts said was a strict interpretation of the global rules.

APRA also said that other asset classes could be added to the list later, depending on market developments.