The comparison website finder.com.au has issued a warning to borrowers to check the "revert rate" of lenders when shopping for a fixed-rate mortgage. It said some lenders were putting borrowers on high variable rates once their fixed term ended.According to finder.com.au, the average three-year fixed rate is 5.07 per cent (according to the Reserve Bank, it is 5.1 per cent) and the average variable rate that borrowers revert to is 5.51 per cent.However, in some cases, the difference between the fixed rate and the revert rate is much higher.Suncorp is offering a three-year rate of 4.98 per cent, which currently reverts to 5.99 per cent - a difference of 101 basis points.St George Bank's revert rate is 89 basis points higher than its three-year rate, Commonwealth Bank's is 81 bps, Citibank's is 80 bps and Westpac's is 79 bps.A finder.com.au spokesperson, Michelle Hutchinson, said borrowers tended to assume that the revert rate was the standard variable rate or the package rate, but that was not always the case.