Green’s Dictionary of Slang

wal n.

(Aus.) a police officer.

1944		Quickmatch: Souvenir Mag. H.M.A.S. ‘Quickmatch’ 42: By this time all dinner was eaten and the ‘Wals’ were asleep again [AND].
1968		J. Alard He who Shoots Last 19: Da Wals packed up young Kenny.
1983		J. Byrell (con. 1959) Up the Cross 19: By the time the wals clobbered him Whiffy Maloney was standing all alone.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 155: The wals also relieve him of his personal possessions which of course include the 173 smackers he’s just withdrawn from his jay-arthur-rank.

In compounds

wal-shop (n.)

(Aus.) a police station.

1996

J. Byrell Lairs, Urgers & Coat-Tuggers 155: [O]ut spring three wallopers who promptly lumber Lennie and the three dipsos and cart them all off to the nearest wal-shop .