Multiline codec in Logstash 5.2.0 not working - logstash

Question

The following codec config in Logstash in never detecting a new line:
input {
file {
path => "c:\temp\log5.log"
type => "log4net"
codec => multiline {
pattern => "^hello"
negate => true
what => previous
}
}
}
Please can someone confirm if my interpretation of the above config is correct:
If a line does not begin with the text "hello" then merge that line
with the previous line. Conversely if a line begins with the text
"hello" treat is as the start of a new log event.
With the above config, Logstash never detects a new line in my log file even though I have a few lines starting with "hello". Any ideas what the problem may be?
EDIT:
input {
file {
path => "//22.149.166.241/GatewayUnsecure/Log_2016.03.22_22.log"
start_position => "beginning"
type => "log4net"
codec => multiline {
pattern => "^%{YEAR}[/-]%{MONTHNUM}[/-]%{MONTHDAY}"
negate => true
what => previous
}
}
}
Log sample:
2016-03-22 22:00:07,768 [3] INFO AbCap.Cerberus [(null)] - Cerberus 'Cerberus Service Hosting - Unsecure', ('Local'), version 1.0.0.0, host 'WinService' 2016-03-22 22:00:07,783 [7] INFO AbCap.Cerberus [(null)] - Starting 'Cerberus Service Hosting - Unsecure' on JHBDSM020000273 in Local. 2016-03-22 22:00:07,783 [7] DEBUG AbCap.Cerberus [(null)] - Starting: WcfHostWorker 2016-03-22 22:00:07,783 [7] INFO AbCap.Cerberus [(null)] - is opening