NSW Transport Minster Andrew Constance says building a new multi-billion-dollar metro train line from central Sydney to Parramatta is the "highest priority for the city", raising questions over when the new airport at Badgerys Creek will be linked by rail.

His remarks raise the likelihood that a so-called "north-south" rail line from Rouse Hill in Sydney's north to Campbelltown via Western Sydney Airport will not be built until some time after the metro line is opened late next decade.

The Turnbull government favours a north-south rail line because of its "city-shaping impact", rather than a line from the $5 billion-plus airport eastwards to Parramatta.

Mr Constance said the proposed metro rail line from the CBD to Parramatta had to be a priority because patronage forecasts showed that by 2031 "we physically cannot get people onto trains from western Sydney into the city" on the T1 Western Line.