How to force contour values in geoshow - rainfall in Australia

This is probably a very simple question. I have plotted a dataset (rainfall) using geoshow with a color bar. I'm trying to compare it with the official dataset via their output and so I want to plot contour lines so I can see if there is a discrepancy. None of the Matlab help seems to cover this, I probably just need to understand some generic idea about map plots, but I don't..

Here is my simple filled colorbar map, where GP2a is the GPCP rainfall dataset, extracted from their .nc file:

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worldmap('Australia');

load coastlines.mat

geoshow(coastlat,coastlon,'Color','k', 'linewidth', 1.5);

geoshow(Lat, Long, flipud(GP2a'), 'DisplayType','surface')

colormap (flipud(parula))

colorbar

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This works fine.

Now I try contours - all good:

geoshow(Lat, Long, flipud(GP2a'), 'DisplayType','contour')

Now I add to show values - all good:

geoshow(Lat, Long, flipud(GP2'), 'DisplayType','contour','ShowText','on')

But I want to constrain them to the same as their website output: 0.5:0.5:5

I tried this:

geoshow(Lat, Long, flipud(GP2'), 'DisplayType','contour','ShowText','on',[0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5])

But got the error message:

Error using parseRasterInputs>checkParamValuePairs (line 99). The property/value inputs must always occur as pairs.

Any help greatly appreciated, thanks!