The veteran ploughman finished sixth in his faithful blue Ford in the reversible class with a total of 353.5 points after two days and Riversdale cropping farmer Mark Dillon was 17th on 284 points.
Mr Mehrtens was lying in fourth on the first day of the grassland ploughing then slipped with a ninth in the stubble to end up sixth overall.
In both events Republic of Ireland ploughmen finished first with Eamonn Tracey taking out the conventional and Jer Coakley the reversible.
New Zealand Ploughing president Ian Woolley — a former world competitor — said ploughing at this level was all about performing on the right day.
He said another strong result from Mr Mehrtens was to be expected.
"Bob would have been pretty rapt with a fourth on his first day. He did pretty well and that’s a good level to be there in the top 10 so that’s a great effort for New Zealand.
"Mark didn’t do so well on the first day [18th], but made up for a bit of ground on the second day [12th].
"As per normal, New Zealand travels the furthest to get to a world event like that and we punch above our weight so we are doing all right."
To crack the top 10 meant a competitor was "there or thereabouts", he said.
"It often comes down to whether you have a good day or a bad day at the office really, like a lot of things and if you get a bad plot you can’t really practise for that.
"In the lead up they had quite a lot of rain which was probably ideal as far as soil conditions are concerned. If the soil has plenty of moisture in it, it’s a lot easier to plough.
Mr Mehrtens finished fourth last year in Latvia and second in his best result in Kenya in 2017.
Mr Woolley said the improvement made by Mr Dillon on the second day would boost his confidence when both ploughmen competed at the next world event in Czech Republic next year.
They qualified for this by virtue of winning their individual classes in the national championships near Hamilton this year.
In Estonia the order of ploughing changed with competitors running over the grass first and then the stubble on the next day.
Mr Woolley said this was reversed because of the way this year’s organisers had their fields set up.
He said Ireland had yet again pumped world class ploughmen out with the latest pair winning both classes.
Mr Mehrtens is considering competing in the conventional class at next year’s national championships and is leaving his Ford and plough in Europe for the Czech Republic contest.
Mr Woolley said he understood this was in the plans and ploughing followers would have to "watch this space" to find out his next move .
"Bob’s been ploughing for a wee while now and started off in the conventional class.
"Perhaps he’s doing a full circle going back to where it all began — I’m not sure."
