Les Ferdinand provided a moment of magic that partially lifted an uncomfortable evening as Newcastle were left grateful for their first-leg advantage in the Orjans Vall Stadium.
When Ferdinand crashed home a ferocious volley just before the break, Newcastle looked on course for a comprehensive first-round victory.
But the Swedish part-timers made them pay for being too relaxed with late goals from Torbjorn Arvidsson and Magnus Svensson. Fortunately for Newcastle it mattered little as the Swedes were never going to score the six they needed in the second half.
But Keegan will not have been over-impressed by the performance of his side as their Uefa Cup ambitions were put in a more realistic light.
Keith Gillespie, in for David Ginola, was one of four changes from the first leg which included Warren Barton making his first start of the season.
Both Halmstads goals were similar, with Srnicek unable to hold shots from Robert Andersson. In the 74th minute, it was Arvidsson who was first to the loose ball, beating Darren Peacock to slide home from a couple of yards. Newcastle's embarrassment increased five minutes later when Robert Andersson's shot fell for Svensson to convert off the bar.
Halmstads: H Svensson, Jonsson, Arvidsson, T Andersson, Smith, M Svensson, Ljungberg (Selakovic, 80), R Andersson, Anderssen, Vought, Lennartsson. Substitutes not used: Nilsson, Karlsson, Wyberg, Nordberg (gk).
Newcastle United: Srnicek, Barton, Beresford, Batty (Clark, 70), Peacock, Lee, Shearer, Ferdinand, Asprilla (Kitson, 70), Albert, Gillespie. Substitutes not used: Hislop (gk), Watson, Elliott.
Referee: S Piller (Hungary).Reuse content