The legislative branch of government is the National Assembly (Parliament), which is comprised of two houses: the Senate and the House of Representatives.
Information on legislation and the legislative process is available in English on the Thailand Office of the Council of State website.
Legislation comes into force upon publication in the Government Gazette (2017 Constitution, Article 81).
The Gazette began publication in 1858.
The Gazette is available online on the official Gazette website (open access) but only in Thai. There is no English translation.
An official English translation is available from 1900 to current in print and microform - the only Australian library holding these formats is the National Library of Australia.
Note that all Thai legislation is dated B.E. for Buddhist Era, which began in 543 B.C.
Laws include several major civil Codes, Acts, Rules, Royal Decrees and Emergency Decrees.
There is no one website containing all Thai laws. The following open access websites contain selected unofficial translations of Thai Codes, Acts, Regulations, Decrees etc as passed. None of the sites appear to be completely up to date.
The Foreign Law Guide (UniMelb staff & student access) lists legislation alphabetically by subject and links to English translations of the laws on open access websites where possible.
The Constitution is the Supreme Law of Thailand. The absolute monarchy was replaced by a constitutional monarchy in 1932, and Thailand's first constitution was promulgated in the same year. There have been 20 Constitutions since, including the 2014 Interim Constitution and the latest - the Constitution of the Kingdom of Thailand (B.E. 2560 (2017). On average, Thai Constitutions last less than 5 years. However, there is considerable continuity in basic institutional structures in the various constitutional texts.
The website of the Thailand Council of State (open access) includes unofficial translations of all Constitutions since 2011, including the 2017 Constitution.
HEIN Online World Constitutions (UniMelb staff & student access) includes every Constitution from 1932 up to the draft 2017 Constitution.
A warning about currency: the following are good sources of information on the many iterations of the Thai Constitution. However, there is a new 2017 Constitution, on which little commentary has yet been written - bear this in mind when using resources written before the promulgation of this new Constitution.
Melissa Crouch and Tom Ginsburg, 'Between endurance and change in South-East Asia: the military and constitutional reform in Myanmar and Thailand' (2016) International IDEA’s Annual Review of Constitution-Building Processes (open access) - note that this publication provides commentary on the latest Constitution.
Panikabutara Coorey, Pornsakol, 'The Evolution of the Rule of Law in Thailand: The Thai Constitutions' UNSW Law Research Paper No. 2008-45. Available on SSRN (open access)
Ginsburg, Tom, 'Constitutional Afterlife: The Continuing Impact of Thailand's Postpolitical Constitution' (2009) 7(1) International Journal of Constitutional Law 83-105 Available on SSRN (open access)
Dressel, Bjoern, 'Thailand’S Elusive Quest for a Workable Constitution', 1997–2007' (2009) 31(2) Contemporary Southeast Asia 296-325. Available on SSRN (open access)
Borwornsak Uwanno & Wayne D Burns. 'The Thai Constitution of 1997 - Sources and Processes' (Thailand Law Forum) (open access)
Oxford Constitutions of the World (UniMelb staff & student access) - contains English versions of the 1997 - 2014 Thai Constitutions, accompanied by commentary on the 1997, 2007 and 2014 Constitutions.
World Constitutions Illustrated on HeinOnline
See also:
Politics and Constitutions in Southeast Asia by 
The Constitutional System of Thailand: a contextual analysis by