To filter routes 41.0.0.0 and 42.0.0.0 from bgp route declarations outgoing from RTR_SYDNEY :
RTR_SYDNEY(config)#ip prefix-list FILTER1 deny 41.0.0.0/8
RTR_SYDNEY(config)#ip prefix-list FILTER1 deny 42.0.0.0/8
RTR_SYDNEY(config)#ip prefix-list FILTER1 permit 0.0.0.0/0 le 32
RTR_SYDNEY(config)#router bgp 777
RTR_SYDNEY(config-router)#neighbor 172.12.123.1 prefix-list FILTER1 out
RTR_SYDNEY#clear ip bgp * soft