Reserve Bank of New Zealand governor Graeme Wheeler has taken the unusual step of defending his controversial speed limit on low-deposit loans in his own opinion piece in the country's largest newspaper.After weeks of public debate and criticism, Wheeler's op-ed in the New Zealand Herald is his first direct appeal to the public since his September 2012 appointment. He has yet to do a media interview and has previously limited his public comments to prepared speech notes for business audiences and formal Reserve Bank news conferences linked to monetary policy and financial stability statements. The closest he has come to being involved in the heated political debate over the bank's limit on high loan-to-valuation ratio loans is his public appearances before Parliament's Finance and Expenditure Select Committee, although MPsheld back from criticising the bank there.Wheeler repeated his warnings about the dangers of a possible slump in over-valued house prices for the economy and the financial system, but in blunter terms aimed straight at borrowers and voters."The resulting stress in the financial system can have long lasting adverse effects on the economy," he said of the fallout from a housing bust."For borrowers, it can mean years of spending cut-backs to rebuild savings. The greatest impact is on borrowers, often first-home buyers, who recently entered the market with the least equity."The political debate around the impact on first-home buyers has been the most intense and cited by some in a recent surge in the opinion poll standings of the Labour Opposition under new leader David Cunliffe.  He has sharpened his attacks in recent weeks on the government's acquiescence to the Reserve Bank move and said Labour would exempt first-home buyers and those outside of Auckland and Christchurch if it became government.For the first time Wheeler also spelled out to regular borrowers the likely increase in mortgage rates over the next two years that is implied by the Reserve Bank forecasts in its September quarter Monetary Policy Statement."We currently expect that the official cash rate could increase by 2 percent from 2014 to the beginning of 2016. This could result in interest rates on first mortgages of 7-8 percent," he said."If the loan-to-value speed limit is unable to slow house price inflation, larger increases in the official cash rate would be required."