Thanks all for your suggestions/comments.
Canvassed the immediate neighbours and they haven't reported/heard anything out of the ordinary or excessive, at least not compared to some of the other dogs in the neighbourhood.
Came home from work at lunchtime today (parked across the reserve from my house) and didn't hear my dog barking for the 30 minutes I was there. Heard lots of others (including the one barking outside last night) but not mine.
Reference other issues/conclusions that people have assumed.
Backyard - medium to large with trees and plenty of shade. Enough room to run around in. Sheltered with both sun and shade. Lots of toys etc, which are changed regularly.
Working hours - as stated, the hours are the exception not the norm.
Social life - one night a week for 2-3 hours.
Exercise - generally twice a day for a total of about 2 hours off lead.
HVCNSW - asked them for advice yesterday, nothing heard yet.
Bottom line - a neighbour complained and, as a responsible dog owner, I want to correct any potentially undesirable behaviours/habits that MAY be present.