The Australian Border Force has been falling short of its sea patrol target by 20 per cent, which has "posed an increased risk" to maritime security, leaked documents reveal.

The notes from a classified Department of Defence briefing state that the navy and air force are having to pick up the slack left by the Border Force, which has 20 per cent fewer sea-going personnel than it needs - mainly because of underfunding.

The Morrison government has strenuously hosed down suggestions in recent weeks that there is any shortfall in Border Force operations. Today's revelations, which coincide with Prime Minister Scott Morrison's highly orchestrated visit to Christmas Island immigration detention centre, are likely to add further turmoil to the heated debate about border protection in the run-up to the election.

The briefing notes, marked "Protected", state that "manning issues" have meant that instead of having an average of 5.6 maritime patrol vessels available each day, it has just 4.6 boats.

"A large part of the problem is due to internal funding pressures within the Department of Home Affairs, including funding cuts for financial year 18/19," the briefing states.