Properties being taken out of the market had reduced supply.
"A lot of landlords have moved back into homes, particularly ones that have come back from off shore. There's also been a lot of investment properties that have been around for a while being sold.
"The housing market's got stronger so it's a good time for investors to get out [of the market]. [Former] rental properties are now owned by families and first home buyers."
He expected rents to gradually increase in coming months.
"I wouldn't be surprised if there's another rent increase next month, maybe not as much as $10, but it should continue to go up. Obviously they'll only rise to a point because tenants are limited by their incomes, but with such strong demand, gradual rises in rent are pretty likely."
Month-on-month, the national median weekly rent remained unchanged at $420, a record high first reached in January. This was a 6.3 per cent increase on May 2014, however.
Trade Me Property head Nigel Jeffries said it was the weakest month for median rent prices in 2015 - "but that's not to say it's cheap. It's also worth remembering that over the past five years the median weekly rent across New Zealand has risen 23 per cent from $340 to $420 and that's much, much faster than household inflation."