Sorry but I have to put in my bits on this "...a native English speaker to fulfill in a post of instructor...": 

1) "native English speaker" can include people from Hong Kong, Singapore, Zimbabwe, South Africa, ... as well Britain, North America, Australia and New Zealand; this specification is racial discriminatory and prejudicial against people (of any country) with competent skills in teaching English.

2) A vacancy (position) is to be filled - whether in part or in full will depend on actual performance on the job.  Applicants should try their best to convince the selection panel of their English Teaching ability at the required level (university). <NB. 'a position of instructor' is used more often than 'a post of...'>

3) Qualification 1 states 'Male/Female'. Does this exclude people somewhere in between by law?

4) Qualification 2 is discriminatory against 'age' and thus is in breach of the UN Human Right Declaration (which Thailand is a signatory) and perhaps in breach of Thailand Constitution too.

5) Remuneration (or salary), other benefits and working conditions are not mentioned at all. (A lot of work can be spared by giving out as much information as possible so people can consider applying carefully.)

I understand the wish to obtain the service of native speakers to teach the language they speak at home. But we must not fail in our duty to observe "the right things". Otherwise, we can end up the same way as as our current government.

;-)