postgis buffer a lonlat point with radius in meters

postgis_buffer.sql

TLDR;
----------------------------------------------------
 st_transform(
    st_buffer(
      st_transform(st_geomFromText('POINT(145.228914 -37.92674)', 4326), 900913),
      50 --radius in meters
    ),
    4326
  )
----------------------------------------------------

// we transform the point from SRID 4326 to GOOGLE UTM mercator
// this means its a grid not a sphere
// we then buffer the point with 50 meters
// the resulting geometry is a point in UTM buffered in 50 meters
SELECT st_buffer(
  st_transform(
    st_geomFromText('POINT(145.228914 -37.92674)', 4326), 
    900913
  ),
  50
)

//we can verify this by envelope, notice the coordinates in meters 
SELECT 
st_astext(
  st_envelope(
    st_buffer(
      st_transform(st_geomFromText('POINT(145.228914 -37.92674)', 4326), 900913),
      50
    )  
  )
)

                                                                                         st_astext
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 POLYGON((16166758.7549401 -4569131.79492395,16166758.7549401 -4569031.79492395,16166858.7549401 -4569031.79492395,16166858.7549401 -4569131.79492395,16166758.7549401 -4569131.79492395))
 
// transform it back to 4326
SELECT
st_transform(
  st_buffer(
    st_transform(st_geomFromText('POINT(145.228914 -37.92674)', 4326), 900913),
    50
  ),
  4326
)

// now we get the coordinates in 4326
SELECT
st_astext(
  st_transform(
    st_buffer(
      st_transform(st_geomFromText('POINT(145.228914 -37.92674)', 4326), 900913),
      50
    ),
    4326
  )
)

                                                                                         st_astext
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
POLYGON((145.229363157642 -37.92674,145.229354527204 -37.9268091193645,145.229328967552 -37.9268755824493,145.22928746093 -37.9269368351258,145.229231602415 -37.9269905235039,145.229163538616 -37.9270345843878,145.229085885188 -37.9270673245611,145.229001626309 -37.927087485853,145.228914 -37.9270942934869,145.228826373691 -37.927087485853,145.228742114812 -37.9270673245611,145.228664461384 -37.9270345843878,145.228596397585 -37.9269905235039,145.228540539069 -37.9269368351258,145.228499032448 -37.9268755824493,145.228473472796 -37.9268091193645,145.228464842358 -37.92674,145.228473472796 -37.9266708805706,145.228499032448 -37.9266044173007,145.228540539069 -37.9265431643473,145.228596397585 -37.9264894756426,145.228664461384 -37.926445414432,145.228742114812 -37.9264126739818,145.228826373691 -37.9263925125049,145.228914 -37.926385704806,145.229001626309 -37.9263925125049,145.229085885188 -37.9264126739818,145.229163538616 -37.926445414432,145.229231602415 -37.9264894756426,145.22928746093 -37.9265431643473,145.229328967552 -37.9266044173007,145.229354527204 -37.9266708805706,145.229363157642 -37.92674))
(1 row)