#!/bin/bash
s=':account_id:12345:6789:Melbourne:Aus
:account_id:98765:43210:Adelaide:Aus
:customer_id:98765:43210:Adelaide:Aus'
 
awk 'BEGIN{OFS=FS=":"} /^:account_id:/ {result="";for (i=1; i<=NF; ++i) { result = result (i > 2 ? $i : $i OFS)}; print result}' <<< "$s"