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IF THE Reserve Bank lowers interest rates next week, which lenders are most likely to pass on the cut in full?

Judging by the bank-bashing that follows most official rate reductions, you might assume the big banks are the most stingy on this score.

If you listen to Treasurer Wayne Swan, you might also assume small rivals to the banks are more generous. Swan repeatedly urges borrowers to shop around for a better deal and has introduced policies designed to turn the building societies and credit unions into a ''fifth pillar'' to compete against the majors.

However, a closer look at recent trends turns these assumptions upside down. Indeed, figures from Canstar suggest credit unions and building societies - known as mutuals - have been just as guilty of failing to pass on official cuts in full as the big four.

Since the Reserve began this cutting cycle last November, Canstar says mutuals have passed on an average of 98 basis points of the 150 basis points in official cuts. Non-bank lenders have passed on 99 basis points. Some smaller rivals to the banks have been more generous than this, of course - it's an average, after all. All the same, this is a tad less than the cuts that two of the big banks, the Commonwealth and ANZ, have passed on to their customers.

In another surprising result, the big bank that has passed on the least is NAB. This is the same NAB that last year claimed to have ''broken up'' with the big four over their shabby treatment of customers - and yet it has passed on 89 of the 150 basis points.

This doesn't mean borrowers should ignore Swan's advice to shop around for a better deal, of course. The non-bank lenders and mutuals still have lower advertised rates than the big four, though the big banks often discount their standard variable mortgages by another 70 basis points or so. The NAB's standard variable rate is the lowest of the big guys.

Clearly, however, the lenders' unpopular tendency to keep some of the rate cuts for themselves is widespread. Shopping around is a good thing - it keeps banks on their toes. But it won't put an end to the slanging match between politicians and banks that follows most of the official cuts.