What really riled Mr Glenn was the "sneaky" way in which the Brethren had reportedly tried to hide the fact it was behind the campaign.
"I thought, `Poor old Labour; let's make this a little more interesting.' There's a little bit of imp in me."
Besides, he believed Prime Minister Helen Clark had been doing a good job of running the country.
She was determined, honest and really cared about the country and its people, he said. "I think you get a fair deal from her."
So he settled on $500,000 - because $100,000 was not enough and $1 million was too much.
In the past, Miss Clark had tried to lure him back to New Zealand and into the Labour Cabinet, suggesting that, with his background, he would be a sitter for the plum role of transport minister.
But Mr Glenn was not convinced, saying that, with all the major transport assets sold off to private owners, there would be little for him to do.
He had not given any thought to whether he would donate to Labour's campaign again this year, or to any other political party, but believed they no longer needed his help.
After the last election, he told the Labour Party to ditch the begging bowl and lent the party "a relatively small amount of money" to employ the services of fundraisers.
"From what I'm told, they got some very good advice ... they should be self-supporting now." The loan has since been repaid.
As for Labour's performance since his controversial donation, Mr Glenn said Miss Clark had done an "adequate" job, having got a free trade deal with China. But now, for the sake of expediency, New Zealand needed to drop its no-nuclear stance and do a deal with the United States.
"Get the bloody thing. Pump another $4 billion into the country so we can afford another beer and put another ten bucks on the horses."
