They had “Milk Cows” - U-boat tankers. They also had covert tenders; the U-boats could refuel from raiders. U-boats had world-wide reach.
http://en.wikipedia.org/wiki/German_auxiliary_cruiser_Orion#Raider_voyage
Here’s my candidate for the minelayer... The German raider “Orion” had been operating in Pacific waters and laid mines off Auckland (New Zealand) in June 13/14 1940. She was resupplied at sea & operated with another Raider, “Komet” nearer to Australia.
She had the opportunity to sew another minefield. Reports don’t say that she did, but she is a likely candidate.
A related article states that the Bass Strait minefield consisted of 100 mines. Probably that was an estimate based upon the subsequent sweeping operation. Anyway, if that is anywere near close to the true number of mines, then this was not a minefield laid by submarine.
http://www.google.com/hostednews/ap/article/ALeqM5iju_ys2226NaH7pQOL70-AZG9p3AD979HT8O0