The Macquarie Dictionary lists the following pronunciations: butter /ˈbʌtə/, apart /əˈpaːt/ & action /ˈækʃən/. Wiktionary lists them as butter /ˈbʌ.tə/ (RP), apart /əˈpɑː(ɹ)t/ (RP) & action /ˈæk.ʃən/. However, to me they sound like butter /ˈbʌtʌ/, apart /ʌˈpaːt/ or /əˈpaːt/ & action /ˈækʃən/. If I try to pronounce "butter" as /ˈbʌtə/ it sounds weird to me although "action" sounds fine either way. To me the /ə/ sound in "action" is the true schwa sound but "butter" has the same vowel sound twice. Also, the difference between the two vowels in "apart" just sound like a difference in length to me: /ʌˈpʌːt/. (I just noticed that http://en.wikipedia.org/wiki/International_Phonetic_Alphabet_chart_for_English_dialects lists [äː] for "palm" and "father" and [ä] for "strut", "run", "won" and "flood" for AuE, so using that my "apart" would become either /äˈpäːt/ or /əˈpäːt/.) Are there any studies showing a split in what is normally considered a single phoneme, or am I just weird :-) ?
I'm majoring in linguistics and last semester I took a class on the English sound system. What we learned is that unstressed vowels reduce to a schwa in English. We mostly focused on General American since my school is in the USA. The rule may be different in your particular dialect. There were some phonetic transcriptions in that class that I had to disagree with since my dialect (Western NY State) was a little different. One example is that every textbook and professor I've known transcribes the "-ing" ending with a short "i" like the one in "it" in many dialects. Every time I say a gerund or present participle all I hear is a long "i" like in Bastille. So, the general rule states that the unstressed vowels in "butter" and "action" will be schwas. Your individual way of speaking might be an exception. Remember also that as you carefully pronounce a word to yourself you will eliminate schwa sounds because you are trying to hear each vowel carefully. Try to record yourself in natural conversation saying these words. That might reveal to you that you actually do say /ˈbʌtə/.