There should be no problem ... just make sure that you apply for your extension before (at least seven days, if possible) your current permission expires.
The only difficulty will be if you are asked for your return ticket on arrival ... but, as with most processes here, I'm sure that a little 'ex-gratia' payment would remove any difficulty.
Oh, another point ... if you do have a visit visa, allowing you an initial stay of 59 days, make sure that the immigration stamp in your passport is marked up accordingly. I have heard of one person who arrived with a visa, but was only stamped for a 21 day stay - he had to pay a penalty when he went to extend close to the 59th day.
I should point out that I did, eventually, receive my 13a (permanent residence) visa on October 2, 2008. This means that I was here for ten months on a repeatedly extended visit.
