Transform GeoJSON geometry to WKB

Use ST_GeomFromGeoJSON.

SELECT ST_GeomFromGeoJSON('{"type":"MultiPolygon","coordinates":[[[[-125045.48351212002,4577567.588141698],[-124816.19981552364,4577552.93014355],[-124765.99472517562,4577419.175847012],[-124842.47121534991,4577392.905406596],[-125045.48351212002,4577567.588141698]]]]}');

Returns:

010600000001000000010300000001000000050000006B3477BC5787FEC0141DA4E54776514134C371320379FEC0D078873B447651411DF264EADF75FEC0D31341CB22765141541B198AA77AFEC0822EF2391C7651416B3477BC5787FEC0141DA4E547765141

To set the SRID (3857: Google Mercator) and transform to EPSG 4326:

ST_Transform(ST_SetSRID(ST_GeomFromGeoJSON('{"type":"MultiPolygon","coordinates":[[ ... ]]]}'), 3857), 4326)

Note that without setting the SRID, PostGIS will "assign" the SRID of 0. Your question did not indicate a projection, but it looks like Google Mercator (EPSG 3857). Your comments indicate you want 4326, so once the SRID has been correctly set, you can transform to that.

This returns:

0106000020E6100000010000000103000000010000000500000074A6F23D0CF9F1BF443A04EE50FE424051E7FD7F9CF0F1BFA74F72874DFE424040E59997C3EEF1BFE64B6F7F2EFE4240A669C2F693F1F1BF8356276728FE424074A6F23D0CF9F1BF443A04EE50FE4240

Which in WKT is:

MULTIPOLYGON(((-1.12330269049053 37.9868447800295,-1.12124300000001 37.986741,-1.12079200000001 37.985794,-1.121479 37.985608,-1.12330269049053 37.9868447800295)))

Use ST_GeomFromGeoJSON

select ST_GeomFromGeoJSON('{"type":"MultiPolygon","coordinates":[[[[-125045.48351212002,4577567.588141698],[-124816.19981552364,4577552.93014355],[-124765.99472517562,4577419.175847012],[-124842.47121534991,4577392.905406596],[-125045.48351212002,4577567.588141698]]]]}')

Result:

"010600000001000000010300000001000000050000006B3477BC5787FEC0141DA4E54776514134C371320379FEC0D078873B447651411DF264EADF75FEC0D31341CB22765141541B198AA77AFEC0822EF2391C7651416B3477BC5787FEC0141DA4E547765141"

Your GeoJSON sample lacks information about the coordinate reference system, although you say it is SRID 900913. This is so-called Google Mercator or Web Mercator, which has been superseded by EPSG:3857, which I will use in this example. An important question is whether you really want Well-Known Binary, which does not include an SRID, or Extended Well-Known Binary, which does. As PostGIS uses EWKB internally ST_GeomFromGeoJSON will produce a geometry with the SRID, but since your test data does not include an SRID, the EWKB will be created with SRID of 0.

(In the code below I omit the lengthy GeoJSON for readability.)

So for example, the geometry can be converted with ST_GeomFromGeoJSON('...'), but

SELECT ST_SRID(ST_GeomFromGeoJSON('...'));

returns 0. If your data are in Web Mercator, and you tried to insert the result into a PostGIS table with a geometry column defined as Web Mercator, the insert would fail.

Nonetheless, if this is what you want (WKB, no embedded SRID), use:

SELECT ST_AsBinary(ST_GeomFromGeoJSON('...'));

If you want EWKB (WKB with an SRID), you have to use the ST_SetSRID function to assign the correct ID. You would then return the EWKB using:

SELECT ST_AsEWKB(ST_SetSRID(ST_GeomFromGeoJSON('...'), 3857));

Note that since PostGIS geometries are all EWKB, the ST_AsEWKB function is possibly unnecessary. But since you haven't specified the use I'm not sure if you would run into data type trouble if you skip it.

Finally, if, as you indicate in your comments to various proposed answers, you actually want the final result to be in EPSG:4326 (WGS 84 lat-long coordinates), you have to apply the ST_Transform function after defining the SRID with ST_SetSRID:

SELECT ST_AsEWKB(ST_Transform(ST_SetSRID(ST_GeomFromGeoJSON('...'), 3857), 4326));