The seat of Penrith - a symbol of the Coalition's success in western Sydney at the 2011 NSW election that brought it to power - is at serious risk of falling to Labor amid a slide in popularity for the Baird government, new polling shows.

A ReachTEL poll commissioned by the Australian Manufacturing Workers' Union (AMWU) shows Labor's primary vote in Penrith at 37.9 per cent compared with the Liberals' 32.2 per cent.

The Greens polled 3.4 per cent, others 16.5 per cent and 9.9 per cent were undecided, according to the poll of 757 residents taken the Sunday night before last.

A two party preferred figure was not reported for this and three other seats polled as the high "other" votes relative to the 2015 result meant the preference flows recorded at that election could not be applied.