Median house prices in the 70s and 80s (from this study:
http://www.econ.mq.edu.au/research/2004/Abelson_9_04.pdf)
From these figures, no evidence prices were affected one way or the other by the 1974 floods.
Table 1 Annual median house prices ($) - capital cities
Year....Sydney...Melbrne..Brisbane..Adelaide...Perth....Hobart...Darwin..Canberra
1970...18,700....12,800...............................17,500
1971...21,200....13,400..................11,900...17,750...11,875..............18,000
1972...23,700....15,000..................13,225...17,500...12,600..............20,350
1973...27,400....19,800....17,500....16,250...18,850...15,200..............26,850
1974...31,800....25,500....21,500....22,200...18,850...20,500..............32,000
1975...34,300....28,700....23,700....26,150...24,500...25,850..............33,600
1976...36,800....32,900....26,275....29,800...33,000...31,575..............35,100
1977...39,200....37,000....28,600....32,600...36,400...34,500..............36,700
1978...43,200....37,600....29,975....33,100...38,575...34,000..............37,300
1979...50,700....38,000....31,450....33,750...38,600...34,750..............39,000
1980...68,850....39,500....35,475....36,000...40,350...36,250..............44,675
1981...78,900....44,000....45,325....39,100...43,825...37,100..............57,750
1982...79,425....46,750....55,125....42,850...48,225...40.325..............59,025
1983...81,425....52,500....55,525....47,950...49,000...42,500..............68,150
1984...85,900....65,000....58,950....61,250...48,175...44,750..............84,250
1985...88,350....75,200....61,550....72,200...52,050...55,500..............90,625
1986...98,325....82,000....63,000....73,500...58,000...56,725..87,500..91,175
1987..120,025...89,500....63,500....74,500...61,225...63,450..81,075..90,125
1988..141,000..109,000...71,000....80,400...78,000...67,950..86,000..101,250
1989..170,850..132,000...96,000....90,400..102,500..77,325..90,750..115,000
etc...